Joined a match on Chaos Moshpit playlist, leveled up to 55, but after I the match ended I was disconnected. Upon reconnecting I was back to 54.
I had played a match earlier where it seemed obvious that a teammate was using an aimbot, but I wasn't cheating at all (I even reported the player).
Is this why I was deranked?
No, if you get disconnected, the game doesn't save the progress for that match (that includes levels, challenges, weapon levels, etc.).
If you would have gotten deranked due to breaking a rule, it would have been much worse.
Retrieving data ...