I believe that the type of derailer housing is the problem here.

This is the old, very stiff 5mm housing that prevents the rear derailer from floating about the cassette properly under it's dual-sprung movement. No bueno.

If the housing were longer it would be less of a problem, but this derailer is from the 4mm housing era and needs more flexibility there.

By the way, did you perhaps use any grease on the cable? What you want for best shifting and strong return action toward the smallest cog is some original pre-lubricated SP-41 housing, 4mm in diameter and of sufficient length for the guide pulley to track the cassette profile.
And the cable guide at the bottom bracket should be plastic, or with plastic noodle at least guiding the cable there.

The 9s-era derailers used modest return-spring tension that relied on free cable movement along the entire path of the cable.