just did the faust romance! umph!!! *fans self*
There is a walkthrough located in the game folder for each released character, so that may help.
There are a couple times when Faust disappears. The first time, you just need to go to the band meeting to trigger a scene. The second time, when he is in Germany, may be where you are stuck. The game counts 3 sessions before he returns. By that, it only checks to see if the day of the week is not the same as the last time it checked (Monday vs Saturday on a normal weekend split session). Maybe your sessions are a full week so it always checks on the same day of the week?
If not, then there is a bigger bug and I may need more info.
I did end up having to go way back to the beginning and start his route over using the guide but I believe that was a bug.
I was trying to pursue him but I noticed that when I got the scenes where he went to Germany and came back and took off again, that it skipped the scenes from "Paranoid" to "Guitar Pick". Needless to say I was confused when Lukas showed up. But the last time Faust showed up in game was when they left to go do whatever right after they came back ( he was "friends" with Robin because I wasn't even given the opportunity to choose) and never came back. I played through 6 months and checked the band meeting, did gigs, checked the shopping and band equipment screens to see if I was missing the trigger, and nothing. I checked on various days so it wasn't the same day problem.