I just randomly encountered this issue while using symbols:
I've attached the file that presents this behavior. Try reordering their depth with the controls on the topbar (they do can be reordered manually on the layers panel).
CrazySymbols.afdesign
I change the order (bring to front,...) on one artboard and AD changes all other same images also (unwanted) Please check my short video:
aD-Layerbug.m4v