Indeed! Last time around, it was because Triumphs are meant to be account-wide, but some of them were accidentally set up to be character specific: and there's certain times at which Triumphs (and other) data is updated, accessing the API is one of those times. In the game you don't notice because these events don't fire when you log into a new character, but if you're actively playing the game we load information about your inactive characters - and that might toggle a state that, under certain conditions, makes the game think you just earned the Triumph. I'm not certain if that's the situation now, or if it's something new. There was definitely a rework of how Triumphs are put together with the introduction of those new "interval" Triumphs: so it may have regressed this issue in the process of those changes, or something else entirely may have happened. If allowed to do so, I'll try to talk on the subject once it's resolved!
If it does have to do with account vs. character state, the workaround should be the same as the last time this occurred - earn the Triumph with all of your characters, and it should go away.