An exploit is a bug in the system, that was not meant to be there. That can be used to give players an advantage.

The ability to fail leves, and be able to do them the next reset was added in a patch

Therefore, it is not an exploit, period. Working as intended, who knows(it was originaly changed to be this way so people who fail can have another chance at their reward), but not an exploit.

If you dont want to do it, fine dont do it, but dont hate on people who use it to level.