In the middle of the prechamber is a deflector ball. It is a shaft that has a swollen spot in the middle and the injector sprays on it and the spray is further atomized.
I presume yours got loose and has shaped itself by a little high speed banging around.
Look closely at the other chambers, This ball shaped protrusion is probably black from soot when in place. If your suspect one doesn't look like the rest get a new chamber and all will be apparant.
Bosch Master, ASE Master, L1
33 years MB technician