I was wondering if the chain had been removed for shortening when you installed that corn-cob cassette.

Might the chain have been routed through the rear derailer cage around the bent chain-retention tab on the cage?

I would have a look to see if the chain is flowing smoothly uninterrupted through the rear derailer cage.

And btw, I am using 9-speed chain on my current 1960's Steyr 10-speed as well as on my very old Schwinn Varsity, Continental and Supersport 10-speeds. These chains especially improve the shifting on all Suntour freewheels, and on Shimano 5, 6 and 7-speed freewheels as well.
All of these bikes get ridden hard in the hilly terrain here, and all are using their original steel chainrings. Shifting couldn't be better.