Item discovered to be possessed by someone wo hasn't let you know about it = attack.
Person with item talking politely and willing to give it to you = stand down red alert, talk and offer reward.
But don't mind me offering explanations that don't support you confirmation bias.
This is exactly what it was.
Barca wouldn't let me give the item to the Zuma. So, when it was discovered I had the item in Barca and wasn't talking to the Zuma about it, they got pissed at Barca.
Thus, not only could the Moot do something about it, and they HAD possession of it, they misused their possession of the item and forced Brom to leave the Moot simply to save them from being attacked.
Kabrinskia on the other hand immediately sought to give it to the Zuma. Quite a big difference.
Only downside was that Brom missed his opportunity to have the Zuma go destroy Malus for him.
