While I wholeheartedly agree that players who enter as an alliance should be able to function as an alliance, this should not have been a surprise as it was noted in the patch notes.

* Please note that while players may enter The Diadem as an alliance, upon entering, the group will not be treated as an alliance, but as three separate parties.
I'm honestly surprised no one brought this up sooner to try and get the developers' attention. With that said, you have my like good sir or madam. I'd like to see alliance functionality in the Diadem.