Hardtail loosening on its own?

Just wondering if anyone has had their hardtail loosening on its own when not completely cranking down on something. I have my vise mounted on the back of service truck so it’s open to the elements but for about the last 8 months or so I noticed when I would try to lightly hold on to a soft material object, the handle would automatically spring back. Just wonder if anyone else has had this issue and a potential fix? I will try to post a video if I can figure out how.

The spindle is over greased. Remove the spindle and wipe and remove the grease from the threads and nut.