I don’t know if this is already know or has been reported before, just pointing this out in case it wasn’t already.
So I’ve encountered this error many times now, where I union something in a place, save it, and the next time I’d open up the place in Cloud Edit, the unions would be invisible. You can still select them, but they are simply not visible, and any Solid Modeling command doesn’t work on it (Union, Negate, Separate).
Lately I worked on a tree which has some unions in it. I made this in a separate place, and eventually put it in the actual place it was meant to, so I was certain I had a backup of it if the issue was still present.
This was the result:
This error happens with any union. I made split wooden planks for docks I was making, and these disappeared too. So I temporarily replaced them by normal planks.