Vintage trem bar won't stay tightened

[wired]

Having this issue on a used BFR L3 I just bought (2019 Shadow Gold). I've tightened it via the hex screw , it's fine for a bit, then works its way loose after using the bar several times. I've had many EBMMs with the vintage trem, never had this problem before. Looking for anyone that's dealt with this & hopefully a known solution or tips. Thanks!

 

[wired]

I know it doesn’t exist without pics, so…

 

[Iperfungus]

I've 3 MM guitars with vintage tremolo and never faced such issue.
Usually, you set the trem bar to your tastes by loosening or tightening the small hex screw and that's it.
What I'm wondering is if the hex screw and/or the threads could have problems.