I have a newish PM9. The dang recoil assembly has disassembled itself. The stupid thing seems to just screw together. I put it back together and it functioned but starts to unscrew itself (the end cap) after about 5 shots. What the heck? Do they just loctite these things? My old PM9 had a solid unit IIRC.
I seem to remember reading that they locktite them with red locktite. The normal (blue) stuff would get soft pretty quickly as the gun heats up. Sounds like they might have forgotten that step on yours?
Definitely red Lock tite and be sure to clean the parts well with something that will remove absolutely all oil or grease before using the Lock tite. Lock tite makes a special cleaner, or Lacquer Thinner or acetone will work in a pinch. Use padded pliers and tighten moderately tight when assembling. You don't want to twist the threads off.
Heat to about 300 degrees in an oven if you ever want to remove the end cap again to replace recoil springs or whatever.