Is Itherian chance impeded by encountering nemesis' or portal bosses?

When i’m in an itherian realm and i find an itherian creature but it happens to be in a portal boss fight or part of a nemesis pack and i can’t catch it, is that just bad luck and my opportunity of catching an itherian is wasted? i feel like i could of had the opportunity to catch it if i had encountered a normal fight instead of encountering that nemesis pack or portal boss.
If i encounter an itherian creature in a nemesis pack or portal fight is this really a missed opportunity?