It does sound like a wearing tappet or cam (both?)
If you have a known good engine for comparison you can check for the net valve lift with the same clearance set on both engines.Open up the good engine clearance to match the one in question if you have to.
Are you SURE there is no carbon build up under the valve. It sounds strange but we did see it happen on a Saito 91. The carbon prevented the valve from fully seating but was sealing well enough for the engine to still run . It is worth checking.
Let us know