Had this problem with Kevins XTs in the Alps last year. Managed to get one of those alloy pretension bolts with a view to replacing the cranks when we got home. Didnt bother as it held fast for the rest of the holiday
That’s what Shimano should supply from new. The alloy bolts have two important benefits over the plastic Shimano jobby, They are solid, so tightening the two pinch bolts, tightens the crank arm onto the (narrow wall) splined tube with the support of the alloy nut, keeping it more rigid. If for some reason the arm does come loose, it won’t just slide off as it does with the plastic nut, who’s thread just rips off.
Hope that lot makes sense 8)