After completing the story and going to my first monolith, my main quest changes to “enter the arena”.
I do a few echoes, and when I come back to the end of time, the quest suddenly gets ticked and I get the exp and gold reward. I haven’t entered the arena (but I have keys in my stash).
Happened on several characters.
PS: yes, I do realise nobody cares about these post-story quests, but that’s still weird so I report it. 