(Again, not sure if this is due to a mod or not! Last time it wasn't, so perhaps the same is for this.)
For the Hatchback M35R design (1926 - 1945) the two back tyres can not be painted.
I had the same issue for another car design but I forget the name of it.

This one is kind of hard to show in a picture because I can't show you that I am clicking it. Regardless:
- https://i.imgur.com/RbWWYNE.png

The front two tyres, and the one on the hood + very back of the car can all be painted, but not the two rear ones.

This does not happen for the Pakket design version, only the M35R. (Again, there are other car designs with this problem, I just can not remember their name.)