As Grimgore, I had painstakingly united all the orc tribes (as you do) and then they all secede at once during a Skull Reef Sea Battle, bar the recently awakened tribes.
Also slightly related: was given defender trait for attacking
the Skull Reef (Pirate Rebels) faction... a clue?
May I suggest that the possible issues are:
1. The Pirate Rebels faction is ultimately rich (they own all the treasure islands).
2. When you attack a treasure island the game thinks you're being attacked by the Omnipotent Pirate Rebels
3. All vassals consider their options for survival minimal and secede.
The attached savefiles are the before and after event.
Trawling comments I've seen this is a bug that was meant to be fixed.
It's not too bad for me atm because its orky.... dey jus' wantz sum ov da loot so deyz get awl fighty.
Let me know if you need any more info, I'm running some mods but that shouldn't be an automatic bug report rejection as I think it would happen in vanilla too. idk