Ok, here you go.
'Heads' was the original file I had trouble with, but like I said in my first post, after saving to cloud and re-importing it back into Affinty Photo, everything works well.
'Heads2' is how far I got before it happened again. Now I expect that after reimporting this I can work some more without any problem...until it happens again.
In case you want to try replicating the problem, here's what I've been typically doing after reimporting:
1. Set brush to Drawing
2. Set width to 7
3. Draw, flipping the documentation orientation vertically (which produces a horizontal flip) multiple times, zooming in and out, etc.