If you think about what you have described the rolling stock must be getting a gentle tug if as you say they run freely. You say it does it with all the locos
Do the wagons run freely if you give them a gentle shove or do they stop fairly quickly. This sort of thing generally means the loco is jerking along although it might not be apparent.
Trouble with glueing the motor in place is that the modern motors don't last for ever and ard not easily repairable so there is a very good chance it will have to come out.
If you are talking to me Mico I am aware of the faults and have repaired many locos for other people including the T9. In over 60 years of railway modelling what can go wrong has gone wrong. Anything can be repaired, eventually. 😉