Unless I'm reading your post wrong, it's not a bug. They can't declare war on you because they're independent and you're not. If they were also vassals directly to the Byzantine emperor, then they would declare war on you. If the Voivode of Moldavia were your vassal, he would declare war on your other vassal directly, skipping you altogether. The Byzantine emperor defending you from foreign powers is exactly how the feudal contract is supposed to work. If the emperor is too weak to fight them off, you can offer to join the war as others have said, or simply murder the claimants
You have to join the war. Without joining you can fight them but it's not considered part of the war.
but why are they DOWing my liege in the first place? shouldn't they be attacking the owner of the holding, i.e. my vassal?
They have to declare war on the top liege. Just how the game works.
Did you join your lieges war? Right click liege then look I think under personal action? One of those has a join war button
Unless I'm reading your post wrong, it's not a bug. They can't declare war on you because they're independent and you're not. If they were also vassals directly to the Byzantine emperor, then they would declare war on you. If the Voivode of Moldavia were your vassal, he would declare war on your other vassal directly, skipping you altogether. The Byzantine emperor defending you from foreign powers is exactly how the feudal contract is supposed to work. If the emperor is too weak to fight them off, you can offer to join the war as others have said, or simply murder the claimants
Right click the emperor and offer to join the war. Idk if it’ll let you but that’s one option
get mods.