Why is texture paint showing a different UV map For some reason my UV map is different between UV editing and texture painting. I'm gonna try unwrapping again to see if that fixes it, but maybe there's some checkbox somewhere I'm missing.
Pictured here is what I see in texture paint mode.

And this is what I see in the UV editing panel.
This came down to a problem between UV unwrapping and my modifier stack. All modifiers need to be applied before unwrapping, otherwise there will be glitches in the UV.
In the question above, I had a sub-d and bevel modifier being applied dynamically, and this lead to inconsistencies in the UV map. By applying both and unwrapping a second time my UV islands began to appear in texture paint.