This happened when I did this same exact thing to my xhp millie..I fixed it by rotating the stop pin until the lockbar engaged. It was super early, but over time and use it now locks up right at 50Not enough. It just did't lock up at all. Both of the M390s I've got have very early engagement but they do have full engagement of the locking leaf. The first blade I swapped into the Ti/G10 handle set started to engage but you could unlock the blade with a single finger pressure, forget any real pressure on the spine. The second blade works fine, although as you can see it's still on the early side, but the M4 blade was (by eye, not but instrument) the same.
Safety safety safety safety safety. I've grown attached to my fingers. Blade swaps need to be done carefully by careful people if bad results are to be avoided...yes, I know I am a broken record. I have had too many finger/hand injuries over the decades...![]()
..this may not work for all of them though.