Since Crimson days ended, I've had a "ghost" Triumph of sorts. This specific "ghost" Triumph is one that requires you to 2 man a Nightfall during Crimson Days. I had done this the following week after Crimson Days had ended, and while the Crimson Days icon was still showing up above Shaxx's head after the event had concluded. As you can see in the Imgur post, under the Triumphs tab, there is an icon indicating an unclaimed Triumph under the Events category. However, upon opening up the Events tab, there is no Triumph to claim. I am fairly certain this issue happened because the Triumph triggered for an event that was technically concluded, resulting in me not being able to claim said Triumph. Thanks!
This is a long Known Issue and Bungie is aware of this issue. You'll have to wait until Bungie fixes it. But don't expect a fix soon as it is not game-breaking.