I have completed all scenarios for RSN and the Valley Veteran trophy did not pop. I got no problem for TSW1 RSN but it appears to be bugged for TSW2.
Did you manage to get the trophy eventually? I'm currently in a similar situation although I can not say whether it is linked with the current situation of DLC's not being recognized. I own both DB BR363 and DB BR155 DLC but currently only the 363 works. I completed the last scenario today but did not get the trophy. I have not completed all scenarios from the 155 yet, but as it is currently not "active" I was still wondered why the trophy did not pop.
if you have any dlc for the route installed you also need to complete the dlc scenarios for the trophy to pop. so if the base game has 5 scenarios you need to do the 5 for the trophy but if another dlc adds 3 ect then you need to do them as well I found many of these complete all the scenarios trophys were very glitchy in TSW 2020 but been ok in TSW2 so far
I hope that's the case, I'll know for sure when the DLC are working again. On Peninsula Corridor however the trophy popped after completing the "base" scenarios with one uncompleted scenario from the baby bullet.
Its possible the way the game was coded some may pop if you have dlc in but I would not risk installing the dlc locos until you have the trophy for completing all scenarios for the route. well I have just got this trophy first time on PS5 version of the game NO extra loco dlc was installed for the route so had just 5 scenarios to do and unlocked right away. After playing over 1,000 hours of TSW 2020 on PS (4&5) with so many trophy's glitching I almost panic when a trophy is due. I have in effect tried to go first for ones that were a bit glitchy on 2020 so I can get them out of the way!