Transparent-bg artboard adds white background inconsistently

[alechp]

macOS Mojave 10.14.6
Affinity Designer 1.7.3

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First, here's how these exported PNGs render in the wild:

 

 

These photos are exported from the *same* artboard:

In case the screenshot of the first export doesn't have you convinced, here's the artboard fill & stroke:
(note the fill & stroke, top left)

 
Yes, the slice export options are the exact same:
(also notice how you can see in the slice preview that Color Horizontal is transparent but the avatar_color_why_no_transparent clearly has a white background)



I feel like this should be stupid-simple. What am I missing?


Also here's an example of these files out of preview/affinity designer so it's evident that one's transparent and one has a white background:

Edited October 5, 2019 by alechp
Adding 2 more screenshots and specifying version

[firstdefence]

Go to Document setup... and on the colour tab activate Transparent Background.

 

Exported to SVG, and shown in Quicklook. 

To get the quicklook display above select both exported svg's then click the icon indicated in the image below. This shows the files transparency, I stumbled on this by accident but it saves having to find alternative methods to validate an SVG's image transparency.

[R C-R]

8 hours ago, firstdefence said:

To get the quicklook display above select both exported svg's then click the icon indicated in the image below. This shows the files transparency, I stumbled on this by accident but it saves having to find alternative methods to validate an SVG's image transparency.

An interesting discovery! Thanks!

[alechp]

@firstdefence Great quicklook tip! Thanks

Changing the document setup did the trick.
My problem is solved (!), but I'm still confused.

Follow up questions
1. Why would some of my PNG slices be transparent (eg. the full / mascot logo) and some have white backgrounds (eg. the partial / pictorial logo)?
2. How do I specify a background color that doesn't appear in slices without manually toggling Document Setup before each export? 

Edited October 6, 2019 by alechp
adding context to follow up q #1