I have seen this situation with both the E and D type 8x8 vehicles and clearly it is a bug.
In the second row of wheels one wheel will not move. Maybe is that the terrain under the mud is out of the map and breaks the truck functionality or who knows.
Not seen with 6x6, so it is an 8x8 repeatable issue. Moving the wheel to the left and to the right shows that is not directional, so in any position at certain moments the truck will get stuck while passing over obstacles and unable to move.
Usually when going past some ridges ( it is not the ridge itself but the out of map situation i am thinking)
Also the whole always on for difflock is not working properly.
I have seen all the time that some wheels will spin at different speed than the other although the awd and diff lock modes are on, so they all should have the same speed.
The whole concept of locking the differential is to make them spin at the same speed. I could record that as well but i think you know already and can easily duplicate it. Doesn't require much than an uneven covered surface to see some of the wheels having different speed than the rest.