yeah, the balance point and where to hold the rifle defo affects things.
By adding mass to the rear, you bough the balance back, presumably making the front flip more (effectively with the rifle rotating around your front hand during the firing clcle). By moving you front hand back, there is now more mass in front of that hand, so less flippy.
obvously a lot also depends on the sate of tune; those setup for less hold sensitivity will be less affected.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.