Changes to texture:SetMask() in build 29600 (8.1.5).
So I just discovered something that as far as I know only first started happening in WoW build 29600, March 5th 2019.
In my latest UI (AzeriteUI) I have round icons on my actionbuttons. Some of these fade in and out. But just today I noticed that all of a sudden the icons were square again, and I hadn't touched that code in months! I tried adding callbacks to update the mask texture after applying a texture, when showing or hiding the icon, when changing the alpha, everything. It simply didn't work. But only for some buttons.
So long story short, fix was to call SetTexture before the first SetMask call. It HAS to be done in that order to make sure the mask will stick now. It doesn't actually seem to require a real texture path, though, using SetTexture("") appears to be enough. If you have a real texture path that works too, ofc. Just make sure you call SetTexture with something in it first.
So if you're going to use
...then do yourself a favor and throw in a SetTexture call first;