AD - not sure why "add" feature is behaving this way

[a2jc4life]

I'm sure this is user error, but I can't figure out what's different between these that are doing weird things and the ones that are behaving the way I expect them to, so I'm not sure what to do differently.

I'm working in Affinity Designer, and I have a hollow circle (a ring) shape that I'm trying to combine, one by one, with other shapes.  (The ring is a solid figure; it isn't an outline.)  So I take that image (previously saved as an SVG, if I recall correctly), I place another solid shape over it, overlapping, and click "add."  About 50% of the time, this does exactly what I expect -- I get a shape that's a ring with something else "sticking off of" it.  The other 50% or so of the time, the entire center turns black, and I don't understand why.

Here's an example of one of these pairs, before clicking "add" (this is what the end result is supposed to look like, although I need for it to be all one shape/curve) and after clicking "add."

[R C-R]

Try switching the Layer > Fill Mode setting.

[a2jc4life]

That did the trick!  (At least for this one; it's late, so I'll have to try the others tomorrow.)  Thank you so much!