It would seem the "auto-dimming" rear-view mirror is junk in 2012+ Passats, or there is lot of bad ones (i.e. VW sourced really poor quality/cheap mirror from some vendor).
I thought for a while the mirror was just broken, but I've now actually noticed it few times to dim rather randomly. Never had such problem with Toyota or Lexus mirrors, those mirror just work, unlike the one in Passat...
Have to see at 10k service if dealer will replace it or will they give some BS that it works as its supposed to... If that is the case, then its probably time to find a Gentex mirror that would fit on the mount.