R C-R

Thanks, but I know how macOS works. 😉 The problem was that I didn't see the relevant menu item (Verweise → Felder) at the end of the list.

@artjourney, just a minor thing but note that "AP" is the abbreviation for Affinity Photo, not Affinity Publisher. (It is automatically underlined in posts & if the pointer is hovered over it "Affinity Photo" will pop up.)
You can use APU or the like as an abbreviation for Affinity Publisher.

Welcome to the Serif Affinity forums. 
If you're referring to the contents of the File Explorer window you get when you click Load LUT, yes, that's annoying and it's an area of Affinity processing that could use some work in all the places where Affinity invokes File Explorer (or Finder, on macOS).
I suggest that if you have several directories with LUTs in them, that you organize them so they are in a common parent directory, and that you add that parent directory as a pinned favorite in File Explorer so it always shows up in the sidebar. Then you can just click on it, and if you need to Search you can Search within it.
You could also consider Creating LUT categories in the Adjustments panel, and Importing the LUTs you commonly use into that category where you can find them easily, rather than Loading them each time into a LUT Adjustment,

The way I understand it is that creating the arc in that manner is consistent with what I would expect to happen, making the pie hole 100% merely overlaps the inner radius over the outer radius resulting in a stack, the arc is still a pie shape until it is converted to a curve but the convert to curve action does not remove one of the stacked radii, it simply keeps it covered and the closed shape merely becomes a closed curve.
Everything is logical and there is no bug as far as I can see.
If I were to make an arc from a pie shape I would convert to a curve and break the nodes at the two end points of the arc'd radius and delete the remaining straight curves.

At least for me that only works in the Pixel persona of AD. I often forget that I need to switch to that persona to do that so I end up editing the document in AP needlessly. ☹️

I tried to replicate this issue but I cannot, without clear instruction to replicate I don't think we can consider this to be a bug.

It would probably be useful to distinguish between open and closed curves in the description of the layer type. Description/tooltip Curve does not say anything about the curve type.

When I open the bug.afdesign file the two curves are not expanded or arcs as in your bug2 file -- they are just simple open curves with one node at each end.
In fact, if I start from scratch & like the OP create two pie shapes, convert them to curves & delete all but a pair of different-sized arcs like in the original bug file, Join curves is not greyed out & works as expected.
I have included bug3.afdesign in which I created two new pie shapes, converted them to curves, removed the unneeded curve segments, & joined them. The history is included so you can see how I did that.
So I do not know why Join Curves does not work on the OP's curves. What am I missing?

@R C-R See image for a visual of @RM f/g insight.
 

bug fm insight.afdesign

In OP's curves the nodes are not end nodes. The curves are not lines with a beginning and an end.
Think of them as symmetric shapes – like circles – of which one half is flipped and lies on top of the other half.

Exactly. That was not the question.

1. I spent an Hour going through Settings and Shortcuts, thinking I had hit something, but couldn't find anything. 
2. I didn't know what it was called - so clearly my search brought no joy. 
3. I did search Affinity Designer why did my Node Tool turn into a plus sign. I watched 10 videos - not one mentioned capslocks.  
 

I’m assuming that the Level Adjustment settings used for each image differ rather than all images using identical Level Adjustment settings hence your desire to edit the Level Adjustment values on page.
In general terms anything sitting on the Master Page is ‘fixed’ so you’d add items there that are common to every page that uses that Master though you still have th option to edit Master page items on page that break that rule.
If you’re preparing the images for your photo book in Affinity Photo, I’d personally add a Levels Adjustment to each image in Photo and then add the Affinity Photo file to the respective pages or if adding the images as a TIFF or JPEG to the Photo frame and then add the Level Adjustment in Publisher to each image so you can adjust the values individually for each image as required.
The ‘issue’ having the Levels Adjustment on the Master Page is that when you make any change to the settings on the Master Page those changes will universally affect the images on all pages that Master is applied to.
It’s the same as having a red logo on your Master page, if you then change the colour of that logo colour to Blue it will update on every page using that Master.
Items nested in the Layer or Layers that have a green vertical line to the left of their icon in the Layers panel are Master Page items…
I fully understand what your expectations are but it doesn’t work that way.
Think of objects on the Master Page as being global and objects on individual pages as unique to that page so any change to a global object will be reflected to all pages using that Master.

I most certainly do not know why it would be useful for the Node tool, since the normal triangular pointer already affords precise positioning of the pointer at its tip.
For brushes it makes sense, otherwise it does not, at least to me.

Yes, it's Windows. I tested for it in another application, but it didn't pop up initially. I don't really use Capslock all that much. (I prefer styles)

Yes,
I've copied the text from the notice that appeared when I was trying to open it after copying it to a different location...
Just trying possible and impossible...  

In your opinion. Other people may see this instead completely differently here!

Oh. I failed to read your mind.
 
I would tell you, but you'll already know it.

I thought that was for brushes but it seems a bit odd that it applies to the Node Tool as well.

You cannot close the curve but you can overlap the end (red) node onto the curve, including the node in the red circle.
To close the curve the end node would become joined to the start node. We cannot join to a node in between the start and end.

Caps lock?

Which? Nathan gave you three approaches to try, so it would be helpful to know more about which of them worked for you.

Therein lies the problem with "secondary" artboards displaying their "own" pixel grid but not adhering to it, causing pixel misalignment although in theory there should have been none.
I'm all for the artboard flexibility as we have it, since meanwhile I precisely understand the issue at hand and thus I can easily adapt.
But obviously there are way too many users that don't know how to deal with it. This is a problem that needs to be addressed by Serif.

Not sure what you mean by that because while a computer screen can't display its pixels in anything other than whole pixel units, it is certainly possible for an app to display objects in documents that do not align perfectly to the pixel grid and/or have fractional dimensions.
Also, do not forget that among other things artboards can contain other artboards, pixel layers made with pixel brushes than can never be perfectly pixel aligned, & so on, like in this simplified board in board.afdesign example. Obviously, this is not something everybody would do but it does have its uses, so Affinity does & should allow for that.