I was actually disappointed at Bayern Munic for loosing to Mainz but as it is football, such things happens to male the sports interesting and full of surprises. On the other hand, I love the energy with which they bounced back by humiliating RB Leipzig with 5:0. This results shows that Bayern Munic losing to Mainz might actually be a mistake, although, it is a costly mistake. Bayern Munic have good opportunity to win the German Bundesliga this season even though they still have many teams to contend with.
Mainz managed to stop Bayern Munich's record as an undefeated team for 3 consecutive matches. but as you said that when it's on the field then anything can happen. And what I like about Bayern Munich, they don't linger in defeat, where Bayern Munich immediately returns to the path of victory.
Despite whatever records Mainz might have had before meeting Bayern Munic, that is not supposed to stop Bayern Munich from collecting maximum points from them because both teams are in different level by rating and by everything. It only boil down to what I already said before which is that anything can happen in football and I guess what we saw are one of those surprises. The only take home is that Bayern Munic did not allow such a loss to weigh them down emotionally as they were able to get their "pound of flesh" from Leipzig.