Tiktok videos dissappearing from collections

Hello! I wanted to ask if anyone knows the solution to this problem, it has been happening for more than a year now, I think. I save some videos in collections, but then after a certain amount of time, some of them just disappear out of nowhere. And I know they haven't been remove-d (can't type this word fully, reddit thinks I'm self promoting), as one randomly came back for a few days, then dissappeared again. Some, I found in my saves, just not in the collections- but not all. I don't know why this keeps happening. Also, I often save from PC if it could have anything to do with it.