first of all this is not a problem related to fgear, even if you attach that object to a regular rigidbody, it would behave the same.
i can not recreate the case since boxbp is not included in the zip. when you attach that object to the vehicle body it’s not kinematic anymore, it’s kinda part of the vehicle body and i do not know how you connected with the dynamic part, it seems like there is a joint there but not sure, need more data and info about this one.