1. there is a main zone in it the flag UnDestr
2. If in the slave zone register "ParentID" of the main zone and enable the flag in the slave zone UnDestr - the flag does not work

So, to clarify: you're saying that, in your specific case, if both the parent and child zones have "Undestr" enabled simultaneously, the damage protection within the child zone fails and things take damage? If so, I may be experiencing this same issue.