T75 Fifth Wheel Upfit - Differential Lock not enabling.

Haven't tried the other trucks, but since the latest patch it's no longer working for me. No additional mods installed. Stock game.

When turning on the diff lock, the wheels look like they lock up, but immediately reverts to normal.

Can someone verify? Will reinstall if my game is broken.


EDIT:
Tried the other trucks:
T65 Fifth Wheel Upfit - Lock doesn't work
T75 Fifth Wheel Upfit - Lock doesn't work
T75 Long Haul - Lock doesn't work
T75 Dump Bed - Lock works
T75 Cement - Lock works
T65 Cargo Box - Lock works
T65 Flatbed Upfit - Lock works

 

T75 fifth wheel has no diff lock by default, it has transfer case lock, which locks two axles together, but no diff locks.

By adding locking differentials it works like this, first diffs locked, then diffs open.