What values does it have in it's '<strHandlingFlags>' line in 'handling.meta'?
Also, in regard to the 'z' value of 'vecCentreOfMassOffset':
<vecCentreOfMassOffset x="0.000000" y="0.000000" z="this one" />
Lowering the 'z' value does not make the vehicle more likely to flip over, in fact the opposite is true, lowering the value will make the vehicle more likely to stay on it's wheels.
If using 'vecCentreOfMassOffset' to test a vehicle's self righting ability, you would want to raise/increase (not lower) the 'z' value to reduce it's ability to get back on it's wheels.
Also, just so you know, the 'vecCentreOfMassOffset' values, in your case only the 'z' value needs edited, can be negative if required.