Why does declaring war on common enemy lower relations?

So, I'm playing a custom kingdom, was nice-nice with Altair and decided to join in and support their war against Tarth (the "join war" option in diplomacy), another kingdom. I expected relations to go up from this gesture of solidarity, however now in my relations with Altair I see: -3 you attacked a fellow kingdom, -3 you attacked a kingdom

So in total a -6 hit to my relations because I joined a war against my friends enemy. Is this working as intended or is it a bug?

::edit:: meant altAr not altair, too much galciv2 lol..