I LOVE it how I waste six turns and then play a diplomacy card against green, and he gets to attack me anyway, after I don't (understandably) support the area he invades. AH, beta-testing. Yeah, I won't be back...

If you look at the little information box, you'll see that diplomacy cards are played after all moves. It's not a bug, it's probably to prevent people from suiciding into a bonus then declaring diplomacy before you're given the opportunity to repel it.

If you look at the info box though, it's clearly played after movement turns.

take a deep breath. maybe there is a reasonable explanation and coolerheads can prevail

coolerheads didnt prevail because diplomacy didnt work the way they expected