STARTED TODAY, JAN 20 2021, LITTLE RANDOM THINGS ALL OVER MY WORKSPACE ARE JUST GOING MISSING, MORE & MORE AS A I TRY SAVE WHAT THE CRAP IS HAPPENING!!!
A commom known bug with unions is that they tend to get corrupt for nothing and desappear from workspace and there is no easy way to recover them.
Some people say it happens with meshes too, but ive never seen.
If the parts in your game are not unions, and you did checked if they are not cancollide false + unanchored (so they will fall down on play), it is most possible that you've got a virus in your studio or in your game. Did you installed any plugin or used any model from toolbox recently?