Corrupted symbols after saving and opening

I've experienced this problem over and over again and I've lost hours of work and had to start over many times. I'm animating a single part of a scene that should have taken a few days at most, but it's been more than five weeks and I can't get it to work.

The problem is: I'm animating a character running across the screen using the asset warp tool. However, almost every time I save the project, when I open it again, the symbols are empty. What I mean is that the symbols are still there, but they're invisible, and all that's left are the little gray circles showing that they're still there. This happens for all the symbols in the project.

I started my project a few months ago at 1080p I believe, and the problem was happening a lot, I'd say half the time I saved. It would also occasionally crash and when I opened the reviver file it would have happened. Also, I'm not sure if this is related, but around 40 puppet shapes would be generated.

I tried starting over with worse resolution, but it happened again. Now I'm at ~400p, and at first the problem stopped happening, but now it's happening again maybe even every single time I save and reopen it. Also, I noticed that even in the document that works where the symbols are visible, some of the older symbols I don't use anymore are all weird, like the frames have been scrambled or something.

Another thing is that the disappearing symbols problem seems only to be happening to imported pictures in the symbols. For one symbol, I drew on the layer on top of the imported picture and when I reopened the document the picture was gone but the drawing was still there.

Are there any fixes for this problem?