There isn't an anti drain-back valve in the pump as such, just the tight tolerance of the Gerotor gears.
I think the path of oil to the sump when the car is standing is through the pressure pump then to the inlet side of the filter so will either leak past the drain back valve or have enough pressure/head to push the ADBV open as flow will be in the same direction as when the pump is running, then through the filter, up the oil galleries and leak through the bearings into the sump so the ADBV will only stop wet sumping if it doesn't leak or is strong enough to hold back the head of oil.
Is it possible that another leak route is past the Gerotor gears and leak through the pump where it mounts on the pinion shaft and into the cam chest and then through the cam bearings into the sump not going through the filter?
Wonder if engines that wet sump have lower oil pressure than those that don't, was told by someone in the know that some X wedge motors have hot tick over pressure in single figures.
Not seen the inside of the crank case but the inner cam bearing housing look to be sealed from the crank area so not a leak source.🤔
