So, I discovered a bug in version 2.5- if screen data has the flag "First Enemy Carries Item" checked, and the first enemy is a bubble, they will carry the item, but when all other enemies on the screen are defeated, another copy of that item appears wherever it's placed in the room. So basically, the room is set up like this:
And this happens:
> >
So the bubble drops the key, and one drops as if the flag "Enemies->Item" was checked.