These kits are awesome. I've got one.
The follower in the mag holds the bolt open on the last round, but when you pull the mag out the bolt will close.
I think there are some additional parts they make as add-on's that allow proper bolt-hold-open. It's never bothered me, and I've got thousands of rounds through mine
Only thing is debris from the ammo can clog your gas tube. Easy fix is to fire a couple 5.56 after the .22 to blast it all out.