I don't think Arsenal becomes the favorite here. Although they are playing well in EPL, it can't guarantee anything for the match in UCL.
You can see Arsenal very difficult to qualify against Porto. Arsenal needs to play until penalty shootout. This indicated that Arsenal isn't so strong, the position in the EPL can't describe how strong Arsenal in UCL. If Arsenal is quite strong, they must be easy to beat Porto.
It is different with Bayern Munich. They looked quite good in Leg 2 against Lazio. Bayern Munich have no problem to defeat Lazio with 3 goals. If they were unable to win in Leg 1 against Lazio, I think it can be the impact of Red Card got by Upamecano. Well, as long as Bayern Munich can play with their main lineup, I'm sure Bayern Munich can beat Arsenal. It is not only about the record, but Bayern Munich squad quality also seems better over Arsenal.
The conditions are still the same as they were and even looking at what Arsenal did against Porto I don't think Arsenal will be favourites in the game they're about to play.
Arsenal's condition in the Champions League is difficult to predict as well as Bayern who are still experiencing problems that they cannot solve properly so by looking at this it is likely that in the match everything becomes very difficult to predict.
Another situation that is happening now is just seeing how they perform in domestic competitions in the near future even though maybe this will not have a big impact because domestic competitions cannot be compared to the Champions League but still positive results in domestic competitions can have more impact on Arsenal and Bayern in terms of their mentality.