It's currently impossible to select the BR423 from München-Augsburg in timetable mode. It's still ingame, and it appears as AI or in Journey mode but it's completely absent in timetable mode. As such you cannot run timetable mode services with it, which means on its home route you're stuck using the other versions, without the correct PIS for the route (or if you wanted to use this specific model anywhere else anyway).
I have no problems selecting the MAG 423 in timetable mode. Are you using outdated or incompatible mods, especially timetable mods? Mods that are not directly related to Route XY, the associated trains or timetables can also cause problems on Route YZ.
Not really got any mods, why do you have two Frankfurt Fulda ones though? And, do you own the Köln one?
Yes, I also own the Köln 423. The duplicates are a known TSW 6 problem which currently occurs with almost all trains on almost all routes. See for yourself what a mess it is..
But the Köln one for you isn't available for MAG. Mind you not as huge a problem in that case hah...I'll see if maybe reinstalling DLCs in different orders changes things, for now thanks