The bolt does not have complete control of the cartridge in a push feed action, the bolt picks up the cartridge and simply rams it into the chamber.
Typically when you push the bolt forward on a push feed action, the cartridge is collected by the bolt face. If you stop before the round is chambered and move the bolt rearwards, the cartridge is left in where it is.
The push feed bolt has an extractor “claw,” but it does not grab the cartridge until the bolt is completely pushed forwards.
When the bolt is pushed home, the extractor moves away from the base of the round until the cartridge is chambered snugly.
The extractor, which is spring-loaded, slips over the rim of the cartridge, locking onto the base.
This allows the empty case to be pulled from the chamber after firing, ready for ejection.