There was recently an issue where the mechanic level will not increase when LTS vehicles are won/staged up. This issue has been corrected in update 1.8
What's causing this?
Limited Vehicles are not being included in the mechanic level calculation.
What can be done?
Update to the latest version of the game and your mechanic level will be re-calculated to represent the correct value.
In some cases this appears to cause the mechanic level to decrease when being adjusted to the correct level. This is a one time thing, and going forward all calculations will be applied correctly.