First thing I would suspect is the washer next to the nut. It's cup shaped to apply pressure against the machined surface of the arbor washer. Perhaps the cup has become flattened and no longer has any spring to it.
If the arbor stops faster than the blade centrifugal force wants to, it will loosen the blade. Are your motor and arbor bearings in good condition? If it doesn't coast to a stop it may be loosening the nut.
I once designed an electronic brake for an older table saw. Applying DC to an induction motor will stop it from coasting after the AC power is removed. My brake initially worked too good, and unscrewed the arbor nut. I had to reduce the DC braking voltage to just slow the motor rapidly and not stop it suddenly. If it stopped too suddenly the blade would try to keep going and it would loosen the arbor nut no matter how tight that I tried to make it.
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