This isn't a bug, but a persisting issue (also in past FM versions) because most German reserve teams are playing in leagues which isn't playable/represented in FM (Regionalliga and Oberliga, 4th and 5th tier respectively). While other playable countries have separate U- and reserve leagues (which works fine in FM), Germany is unique in the way that the reserve teams are part of and play in the German football pyramid and is able to get promoted all the way up to the 3. Liga (Tier 3).
An example in FM23 is Borussia Dortmund II, they currently play in 3. Liga (a playable league) and they get to play all the fixtures in that league, giving the players match experience, development etc. If you then take 1860 München II, which plays in the Oberliga (5th tier, not a playable league), they don't have a single fixture because the league they play in isn't represented in FM. This makes the German reserve teams quite pointless and useless, despite being repesented in the game. For some reason the function to let your staff arrange friendlies for the team when no other matches are scheduled doesn't work either, and only 5'ish friendlies in the summer break ever gets planned. You have to manually arrange 30+ matches for the reserve team if you want your players to get some match experience, which is quite a chore.
There's a few possible solutions to this issue, which would make it less broken to play in Germany without custom databases:
1. Introduce the German 4th and 5th tier leagues to FM (Regionalliga and Oberliga)
2. Create reserve league(s) for German II teams, similar to other countries (this would not give realism and would mess a bit with german clubs which do have their reserve teams in playable leagues)
3. Fix whatever it is that prevents staff from arranging friendlies every week, when reserve teams have no fixtures (only 5 friendlies are being arranged in the summer break as it currently is)