>>>*Cam timing has no effect on the adjustment.

Note there are two different length adjuster screws available, it is possible it is as simple as one could have been changed.

Valve installed hieght could also be the cause, or weak spring tension can allow the valve to rotate rapidly, this means wear and seat recession.

A cylinder running hotter than normal can collapse the head of the valve, common with refaced valves which is why we never reface them.

A misground cam lobe could cause this, common with regrinds and very rare with new. Base circles should be reasonably even, 4-5 thousandths is no concern, more than that and the grinder operator needs to be chatted with.

Also a rocker pad that has been refaced a bit too much, wear in the rocker shaft underside or rocker arm shaft bore.

All those things can stack up to be quite a bit....*EB