Is this the behavior of a broken mosfet?
Often a punch-through failure on the gate due to ESD can cause the gate to fail with some voltage on it leaking through from the body.  If this is ESD damage, then handling the FET according to safe ESD practices will prevent future failures.

Keep the FETs in ESD safe packaging until needed.
Use a wrist strap that's correctly grounded when handling the FETs.
Do assembly on a bench with a grounded ESD mat.

Also, some newer FETs have gates that are only rated to 8V.  Make sure your FET gate is rated for 12V operation.