My guess and it is simply a guess, is a tight fitting slide and a bit of whatever jammed in wrong place. My shield 9mm had an extremely tight slide fit and would jam even after smith replaced extractor. Jam not like your jam however, but rather expended brass left in ejection port.

I simply hand cycled slide fully to rear and eased forward many hundred time. Also periodically put on more lube. There was visible wearing in and so far for about 500 rounds no problem.

Now with a glock I had a more similar problem to yours. Slide jammed tight on new gun, I broke loose, cleaned and lubed and several thousands of round later no problem.