I think the "bug" can be explained very simply....
One of your allies (who was thus not the leader of the alliance) made peace with the Mamelukes (who were also not the leader of their alliance) ... At that time, the war get's "split up"..
your (former) ally will be in a war with Oman (and any other allies they might pick up)
you, and the rest of your alliance will be at war with Oman (+remainder of alliance).
the Mamelukes will be at war with your alliance (minus that one ally).
One of your allies (who was thus not the leader of the alliance) made peace with the Mamelukes (who were also not the leader of their alliance) ... At that time, the war get's "split up"..
your (former) ally will be in a war with Oman (and any other allies they might pick up)
you, and the rest of your alliance will be at war with Oman (+remainder of alliance).
the Mamelukes will be at war with your alliance (minus that one ally).