My browser / operating system: Windows NT 10.0, Opera 63.0.3368.107, Flash 32.0 (release 0)

sometime for some reasons, all of the block decide to go to the same place, and I can't just undo it because it only happens when I'm not on the sprite (I'm on another sprite or in the image/sound editor).And it always happens on the same sprite

Happened to me too. Unfortunately, I have no idea what might've caused it.

-8-8-
probably