Michael Cunningham

A line object does not become invisible when it gets pinned inline, it just does not move to a text position by itself – like a shape does for instance.
If you move the text frame you will see the pinned line. In case the line was below the frame before pinning you also can scale the frame height to make it visible.
However, even if I can workaround this by positioning the line on the text before pinning the major issue is that its position can't be set with the options in the Pinning panel. That at least appears to be a bug. And its moving + scaling when unpinned is another bug, which affects not only line objects but shapes. too.

Ah, I see, that makes sense now.
Well, it makes a bit of sense of what I am seeing but it doesn’t make sense in what it does.
It is making space for the line in the text but it is not moving the line to the space in the text and you can’t – while the line is pinned Inline – change the position of the line relative to the text. (In one case I can actually move the text, which seems weird, but the line won’t move.)
Yeah, I’d say that there’s something wrong with that. Pinning a line probably just got missed out – pinning came quite late to Publisher.
P.S. You can pin a single-cell table with only the bottom border stroked, instead of a line, into the text Inline and it seems to work okay (under very quick experimentation).

Another approach does not use pinned objects but text style options only.
For instance horizontal font width:



With this way it is useful to start with a few underscore characters since a higher value for width influences its space characters before and after.
(probably a bug)
 
Another method uses 1 tab for each line and its feature of leading characters which auto-generates dots or line to fill the distance:


underline 5 tabs.mov  

Michael, you can follow this workflow to get what you need for form underlines:
* Create your text as normal with an underscore character where you want the spaces to be (just so you know where to put the lines as you’ll be overwriting the underscores);
* De-select everything;
* Draw a horizontal line elsewhere on the page and make it longer than you will need (make it 1pt thickness if necessary);
* Right-click the line and choose “Rasterise” (very important or the line will not be visible in the text)
* Menu “Edit → Copy”;
* Double-click the text frame;
* Select an underscore character;
* Menu “Edit → Paste”;
* Repeat as necessary.
See Pinning in the Help for more information.
To get bits of the text to stay together use a Non-Breaking Space.
To change the length of a line (see attached GIF):
* Select the line;
* Choose the Crop tool;
* Crop the line as necessary.
Don’t use the handles to resize the line as that will also make it thinner/thicker.
Does this get you what you want?

Hi Michael,
Welcome to the Affinity forum.
If you purchase through the App Store the free content can be claimed through the in-app welcome screen.