Looks like the the extension doesn't want tighten fully. I tested on my WW F-16 grip and it tightens okay, maybe the extension has a bit different thread profile. But it seems the adapter collar also needs an increased thread depth.
Would you be able to make spacer/washer under it? I could mail you one or new collar, but that would take a while. If you need an STL to print, let me know.