Doing runs they couldn't have otherwise done?
It's minor advantage but it doesn't seem unreasonable to do the following:
Wipe weekly runs before the time of fix as it was quickly fixed, don't remove any end of dungeon reward but wipe them from leaderboard / counting for weekly (or just make it count as a 10). The advantage they gain is bonus AP from the weekly, which is measurable.
Possibly ban those who can clearly be seen to changing comp and doing runs far above their regular normal for 6~ hours (believe that's minimum). The important thing here is it shows actions like this will be punished and it would put people out of standing for the next M+ invitational.