Are you still experiencing this issue on the latest version of the game?
I am on the main 1.7.0 version on Epic. My issue is that the faction I join as a merc/vassal declares wars against everyone, literally. I had it in my two campaigns now, as Aserai and Sturgia. In the Aserai campaign Unqid declared war on everyone, even the Sturgians which we don't share any borders with. With the Sturgians, we keep declaring wars, Battanians - Vlandians - Khuzaits - All Empire factions, even though we are getting pummeled at all fronts, losing most castles on the frontlines, but still we keep declaring wars, with 100% support, the kind you can't even affect with 150 influence. It is really frustrating and makes it impossible to make any progress without some sort of exploit, such as the retreat one.
Ah, and the random reinforcement issue is just driving me crazy when trying to hold a defensive position, or attacking a defensive enemy.