This situation appear only once(be noticed), the German King (the attacker) is my liege (AI), under age, I am the Duke of bavaria.
Before the outbreak of war, He(non House Karling) just inherited the Germany King, from his grandfather (House Karling), Kingdom name just change from the kingdom of East Francia to the kingdom of Germany.
The war is to recover the de jure territory (county), belonging to one rebel participant, rather than the rebel leader. At that time, there should be no simultaneous raiding, I can't remember what types of rebellion (perhaps lower crown authority or someone's Bohemia claim).
After the declaration of war, immediately hostile to all rebel, adventurer, neighboring countries. I restart the game, the issue still exists, after the war(Germany win), the issue still exists, and again I restart the game, the issue disappeared.
Hope useful, I can't remember all details, ironman mode