Seat recession, valve tuliping, or valve stem stretching (even breaking) is a known problem on older airheads.
But another problem is that the posts the rockers sit on creep inwards.
They are a press/fit steel bushing in the alloy heads and over time they tend to do so.
So check the height of those posts on the affected side, and compare with the other, which look ok.
I had similar problems with the heads from my old /5, should have taken a few pic's for posterity, but they ended up in the bin before I thought of that.
Geht net, gibbet net.