Vintage trem bar won't stay tightened

Having this issue on a used BFR Luke 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!