Hi Darchon
Before modifying it again, make a backup copy of the prefab.
I suppose you tried to erase the part that you don't want using: Press "z" in one corner of the stone piece, then press "z" again in the other corner of the stone piece, all that selected part will appear.
Now hold down "Shift + G" to accommodate the selection you have made with the arrows that will appear, when you have it selected press "J" and that part will be deleted.
Do the same as you did, update the prefab limit and finally save the changes.
Check that everything is ok, entering the play test option.
Regards
P.S. This prefab is vanilla and you just edited it?
You can still share your prefab and in the afternoon I could test it.