This appears to be a problem between MSFT and HP!
I noticed it last summer on one Win 10 machine and an HP printer... For some reason my wife's PC works fine, and mine mysteriously lost the ability to print auto duplex. I worked every step I found everywhere on the web - nothing. Including reformatting my HD!
I bought a new PC - and guess what, same issue. So I replaced with a Canon printer.
I went to family's house where I set up an HP printer a few years back - and auto duplex always worked fine. And now, guess what.... It's all "Manual" duplex somehow.
My theory is a Windows update in summer 2019 started made it stop playing nice with HP printers.
And everyone keeps saying "If you just do [this] it will be fine." And it never is!