A relatively common problem.
It's caused by the screws bottoming out in the holes. You're not establishing a tensile preload under the head when you torque screws that are bottomed out, you're establishing a compressive preload at the very end.
Problem is, a compressive preload in this application is totally useless.
The fix? Simplicity itself. Find out if/which screw(s) is/are bottomed out with a thin feeler gage (even .001" is too much, so use the thinnest gage you have.) If the gage slips in under the head, remove the screw and add a washer or two. Reinstall the screw and retorque.
Typically, the screw fails (due to a lack of preload and repeated shear stresses,) the head breaks off, and you're stuck with a broken screw shank. Then you have to drill and remove it - no fun.
Even with the full-length "Bombproof" engine mounts, check under the screw heads before you pronounce the job "done." Same thing - if you find any heads with room under them, install a flat washer and retorque. Easy.
If you see space under the head with your naked eyes, install two flat washers just to be safe.
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