UPDATE: My problem is solved, although I don't know why it happened suddenly since I'd made no changes in my system settings.
Solution: Settings --> Network & internet --> WiFi --><Name of my WiFi network> --> Private network
It had been set to Public network (Recommended) which MS says to do for a home network. When I changed it to Private network the printer no longer disconnects when I power down the computer. Now...I don't remember how or when it was set to a Public network, but this is how I found it. The setting worked OK until a recent Win 11 update. Now my network is a Private network and I'll keep an eye on it after upcoming updates. Hope this helps someone.