The bolt hold tab on the magazines doesn't rise enough to engage and lock the bolt back on an empty magazine. Tried it on 858 with DLask mag release and a bone stock 858 Canadian edition of a buddy`s. I'm going to try shave the top of the follower a bit and clearance the tab window to try and get things to work.
Turns out there is enough tilt in the follower to not engage/keep tension on the magazine's bolt hold open tab. Original metal mags have a long skirt on the front of the follower to reduce tilt.