No, the thing is, it is not resetting even if you do (in my case it didn't)...
I think it can happen if you switch to "total" offline mod, and then remove the saves.
Apparently, arena level is saved on the cloud, in a similar manner like the achievements.
You can try to reestablish connection to Uplay, once you will it should change to whatever arena level you were. But again, if you disconnected from the server to prevent updates...