Not a bug. As a ruler, you are only allowed to join a vassals war when they are a part of a Crusade, Jihad or Great Holy War (depending on religion, and which are declared by the head of said religion).
This makes you unable to join a vassal who declared any kind of war (including a regular Holy War) against someone else.
This makes you unable to join a vassal who declared any kind of war (including a regular Holy War) against someone else.