Loosening of the grub screws especially in the drive drain seems to be common to almost all brands.
Part of the problem I found, is that the cub screws used often are of a cutting ring type at the low end. This does give only a very small contact surface on the shaft normally hardened. If there is only a tiny wear it will start to tumble and loosen.
The type of cub screw used for that application should have a flat end.
Grinding the end flat and using Loctide 290 (green) will cure the problem, but you need good tools like Hudy to get them out again.