Hi Cosmics, did you checked whether your lines changed like I described?
Until now, I did not had a case where just one or just a few busses took weird routes – it always affected the whole line that was weirdly changed.
For me it is no option to abandon one transportation vehicle/type completely, as I love to try different things. Because of this, I have towns with bus lines only, towns with tram lines only and towns with a combination of both types. Extremely interesting, how they behave differently.
Recently, I wanted to build an intercity tram line as well, but (at least without additional mods) the tram has a speed disadvantage over busses, so I built an interurban bus line …. which came up with a different challenge than normal bus lines (capacity).