Playing on hard/hard as Arkhan the Black. Once a Military Alliance has been negotiated with the Greybeard's Prospectors dawi faction, they will propose that you join a war they have entered, and thereafter the dawi hold it against you for joining their war (in this case, Sudenberg, -5 penalty)! This has occurred in past patches as well, with the Bretonnians like Thegan's Errantry. Save file attached.
Also calling out another bug which occurred in multiplayer (and so I posted it there), but is more of a battle bug:https://forums.totalwar.com/discussion/250428/bretonnia-dancing-around-central-point-on-map#latest