I had the same with Campy square taper cranks on a Tifosi cartridge BB. Torqued up correctly the bolt kept coming loose. I used cut up feeler gauge to shim 2 adjacent sides of the square and used threadlock to hold the bolt in. What have you got to lose?
Beer can shim. Had a set of Campy square taper in much worse condition, and a small section of aluminium when fitting made it snug for ages.