Rebinding the X, Y, or A key to Helmet Light / Mark results in not being able to exit the vehicles (X), switch weapons (Y), or switch seats (A).
(This is because rebinding Jump, Reload / Vent / Interact, or Switch Weapon / Drop Weapon doesn’t rebind for vehicles.
For example if I rebind Jump to the B key, when in a vehicle I would still need to press A in order to switch seats. If however A was rebind to Helmet Light / Mark, you won’t be able to switch seats.)
When in the driver’s set of the Mongoose, if you switch seats to the passenger seat and then switch weapons, switching back to the driver’s seats will result in hoping off the Mongoose instead.
(May need multiple attempts to see this as there seems to be some timing involved.)