I was wondering if there is a rule that I missed saying that minions/barriers can't have more than one immunity abilities. For example, when I cast Astral Armour on a minion with combat immunity such as Ancient Ghost, it losses combat immunity. However minion with quick or elusive can gain spell immunity without losing any of their abilities. Same for Underworld Elixir, minions with any abilities can gain elusive without losing any previously one they add.
Is this a bug (minions/barriers losing combat immunity when casting Astral Armour) ?
In the advanced rulebook discussion, the Goblins made it clear that it was intended that a creature could have only one immunity. And this makes sense, otherwise certain creatures become extremely hard to kill and imbalanced.