The vehicle manager doesn't fully update the classes section within itself. It only works as long as the number of vehicles in the convoy is the same or less from before replacement.
I've captured a video showcasing the issue. At first I have a train with several carriages all one class. I remove the carriages and add a different set of carriages, whose passenger numbers are reflected in the display. Once I add more carriages than previously in the convoy, the displayed number of passengers remains the same. I then add carriages with other classes, which are also not reflected in the "Change prices" display area.
video (https://i.imgur.com/0X2v4Eo.mp4)
This may be known, but I thought it may be best brought to attention now that the instability bug has been squashed.