Bug? Unknown mechanic?

[Karpad]

So, in the spy/coup mechanic, I just had an odd outcome of a successful coup. I attempt a coup in a city state, which succeeds. My understanding is that I should become the ally of the city-state with the standing of the player I displaced as ally. Instead, the result is that a third player becomes the ally, and in fact is a player I'm at war with, so the city state immediately declares war on me!

Am I missing something?

 

[Browd]

Can happen if the AI immediately drops sufficient gold on that CS on the turn when your coup succeeds. Bad luck, but unlikely to be a bug.

 

[Karpad]

That would make sense if for example that happened on Rome's turn or during the City State's turn. However, when I conduct the coup during my turn it brings up 2 messages:
"Melbourne has declared war on you"
then
"Recruit Citra has staged a coup overthrowing English Supporters to upt a group in office who are allied with you."

I'd think that if the AI dropped cash, that would show up during the AIs turn as the other such notices do.