PICO-8 v 0.1.9.

I'm using sprite page 3 in my game. I was cleaning up some mockup art by pasting an all-black sprite over the top of the mockup in the sprite editor, but when I flipped back over to the map editor and moved the viewport around, the sprites reverted:

This happens every time I repeat this sequence. The sprites revert if I paste over 1 or all of the mockup.

This doesn't happen when I use the pen tool on the sprites in page 3, nor does it happen when I paste an all-black sprite onto a sprite in page 0: