I completed the "Level 6 my second carousel" mission.

I slowly but surely got level 6 earlier each game, just to see if this mission was actually possible to complete. Turns out, I had to get level 6 before the round 10 fight started. This is counting the first three minions, which means this was stage 3-1, a full THREE STAGES before the second shared carousel, and also either 4 or 1 levels (depending on if the first minions count) before the "clarification" that it's round 11 (which only makes sense if you DON'T count the first three minion stages).

Either that or Riot fixed the issue. I hope it's the latter.