My DM rolled over 25 natural 20's in our D&D game

5 in a game, is unheard of, but over 25 is insane. He did say his cheat function was off, but come on! 25 natural 20s? Also, that not including the advantage rolls where he rolled double 20s! There has got to be a bug. Unfortunately, I do not know his browser settings. I wish I did. I would love to roll over 25 nat 20s. I feel like this is something that should be looked into.

You should use the Help Center and Send a request to get Roll20 staff attention and a ticket to track your issue.  It's mostly community support here. 

Hi John! I'm not denying you saw something on the far end of the probability curve, but strange things happen.  Until someone can show repeatable evidence of a bias in the die roller, there's nothing anyone can really go on.  The die roller gets challenged about every six months or so and has yet to amount to anything. By the way, what "cheat function"?

My guess is the "cheat function" thing was a joke.  Couple questions, did you see a "atomic symbol" next to the rolls or, when hovering over them, in the tooltip for the roll?  If you did then the roll should've been a proper roll.  Is your DM using DnD Beyond with Beyond20 extension? If so then the rolls were not Roll20's rolls. They were DnD Beyond's.  Assuming it was a proper Roll20 roll, then as Keith said, it is possible just far end of probability. Out of millions of rolls someone is bound to see it sometime.  It is worth noting here that if you are used to physical dice, physical dice tend to be less random than you might think. They are misshapen and tend to roll 10-11 by the design of most dice manufacturers.