Extremely difficult to reproduce reliably. However, I noticed it happened sooner if I import a PNG and then copy that sprite (from animation editor with ctrl + c) to a new sprite, and then delete the original. Also, fullscreen seems to affect how soon it happens.
However, if I save before I run the app, the next time I open the cap and run it, doesn't happen right away.
I'm still seeing if I can make a cap.