If a player has a stack of, say, six rings of Occidental mages, and then applies the stack of rings to wear one, all the rings will get the same effect from the apply event script.
If these were all separated and applied not from a stack, they should all get different effects. Makes sense that the stack would do the same.
Don't know if this is a problem that should be resolved by changing the timing of event_apply, or by handling a stack of rings safely in the actual event. Probably the latter, I'd assume.
Fixed in maps revision 20809, thanks for the bug report :)