Sorry I was a bit wrong about that bug - it isn't related to "normal" strategic movement cost calculation but caused by changing unit formation & usage of transfer between unit.
Steps to 100% reproduce this issue:
1) Change formation from basic leg infantry to mechanized one. Everything is still good (downgrading formation and releasing trucks work fine too!)
2) Add lacking APC via transfer* from SHQ (SHQ having more unit in reserve then just APC).
3) Strategic move of this unit is completely broken. It's actually broken - unit that normally should have ~300 strategic move cost now have 1432 cost and refuse to move through road still having 968 logistic points available.
*I'm quite sure that it will trigger without manual transfer too - in next turn when regiments will get reinforcements from SHQ. As I already saw this issue previously and this is my first game when I actively experimenting with transfer between units ^_^
Problem will fix herself at next turn. But persist in turn it's triggered even after loading game.
Sorry again for wasting your time. Unsure if you will need a save, but
here is my save. OHQ of 2nd Mechanized Siege Infantry brigade in my capital have 1432 strategic movement cost.