[ByTheLake]

If you have to back out the adjuster screw this much, that suggests the valve is quickly receding. Hard to imagine what else it could be.

Perhaps there's accelerated wear of the valve seat or the valve face or both?

Were these valves ground or lapped in recent memory? If these valves used a hard plating like stelite, lapping or grinding could remove this protective surface, accelerating wear.

It doesn't sound like an issue with the adjuster working its way loose.

Regarding your comment about the "insides" ... each valve has one rocker arm, and each works independently of the other. Adjusting the valve lash on one can't directly affect another.

If it was my bike, I'd pull the head for inspection and measurements to learn more.