This has also happened to me. The odd part was I was in another zone later that day by myself with no tags, and a coffer popped for the union that I did not receive prior. I was unable open it.
@Celestinia - I've GMd that, asking the same thing. According to the GM, that was purposefully done. It's not a bug. The GM couldn't divulge information, but from what I could tell from the conversation, getting a coffer to appear for your union depends on how well your union does as a whole.
@Johnyboy - It doesn't matter who kills the mob, it depends (I believe), on simply how much xp as a whole your union gets.
only thing i can think of, is if your Union, whether it be just you, or you and 50 other ppl, does not kill the mob, u don't get any loot... i.e. "Johnboy defeats <t>." If it was not ur union that kills the mob, then u don't get the treasure from it. to earn a claim to so and so union's spoils, all you need is to earn 300 or so exp., but to actually get spoils, you have to kill the mobs. that's from my experiences anyways.