Having witnessed a number of shooters having ‘difficulty’ achieving zero and then holding it, the problem has always been a lack of awareness in how to setup both the front and rear base if using high-quality German QR rings. As you’ll be aware, there is a locking grub screw that ensures the forward tensioning screw; the one that determines ring spud pressure, stays in place at the desired setting.
This would be one of the screws you’ve used to disable the QR feature, so readers can well imagine the impact on accuracy if it is loose. I’ve even seen mounts where the locking grub screw was missing.
K