Yes, but, as said, by a pure objective logic, random is, that you have not declared war on Armonia as well.
They had made, in terms of military aggression, movement and diplomacy the exact same things as the Westmoorians (till your war declaration).
The Oberndorf issue etc is a thing between Nivemus and Westmoor.
If you declared a war due to the things between you two, then this has nothing to do with Perdan or well, Armonia. Perfectly ok then.
But you Ihope you have thought this through before, cause now, if there should be peace between Perdan and Nivemus, the war between you and Westmoor will be excluded.
I don't know how many other ways I can explain it.
Tensions between Westmoor and Nivemus have been building for some time. Westmoor is probably Nivemus' biggest rival(due to diplomatic, strength, and self-engineered circumstances). We can gain something from warring with them and vice versa. Also, their nobles can just waltz around our land if they want if I hadn't declared war.
Armonia is not our rival, in fact, until recently, they have had no influence in the North or in Niveman policy period. We don't really gain anything immediate from warring them, and they don't really gain anything immediate from warring us. And they would be hard pressed to waltz around our land due to their capital being so far away.
I declared on Perdan simply because your amount of infiltrators and scouts were too much for me to ignore, so, in that war, the logic behind the pros of going to war with Westmoor and Perdan overlap.
So, you see, there is a perfectly logical reason for the my diplomatic relations... Not that it matters, because since when did diplomacy ever have to make sense(both in real life and in battlemaster)?