I suggest you look everything over carefully.

I highly doubt the valve has moved .100”.

Did you have the valve lash shrink from whatever you set it at, to zero...... 4 or 5 times?

My guess is, there was a measurement error previously, which had the installed height set way too tall...... which is the root of the whole problem.

I question the 1.975 too though....... since it’s not always easy to get there with std length valves in 906 heads.

I’d pull the heads, inspect the guides and seats, and start over from scratch with the spring set up.

If you’re not using a height mic....... you should get one.