I just completed the raid on Master with some very cool guys find with LFG since it was the last triumph I needed for Rank 11. But to my immense disappointment once I left the lobby I noticed that the game didn't count the completion. I don't even have the checkpoint anymore and on Raid Report it actually shows as completed correctly. It's absurd that something like this happens, especially with the most difficult triumph along with the Solo Dungeon one.