Happened to me too. Item placers were the most common thing deleted from my map. I also had other objects deleted, about 30-50 total. They will typically delete during the memory check. I have captured them actually disappearing on a screen recording.
At one point, I put down an Item Placer, hit save and it simply vanished. Completely gone from the entire project. Put another one down, vanished again. So I just left it. Ran memory check and tested my game, the item placer was actually there in the private map during play testing but nowhere to be found in the outliner or in my local files on my computer. I can’t figure out what is going on.