First mulligan should always be free
You've done this in special events before, but it is such a reasonable solution to your on-going issues w/ the defective deck shuffling algorithm that it would go a long way toward reducing some of the dissatisfaction with how massively broken the shuffler is that you should consider making this a standard protocol for Arena. I don't care if that varies from standard play, since real cards don't suffer from the broken randomization issue that Arena does...
At least until you get around to fixing the broken shuffler. GET ON THE STICK!