SOLVED - I tried rolling back the driver for the NVIDIA display adapter, and that worked.  I don't know enough about computers to know why it worked, but it did.
Windows 10 Pro XPS15 9500 connecting to an LG 32UL500W.  Laptop and monitor both support 4K UHD (3840x2160) display, and usually I have no problem.  However, if I unplug the LG and connect my laptop to another HDMI input (e.g., to lead a zoom meeting in another room in the building), when I come back and reconnect, my LG displays a max 1920x1080 resolution.  Device manager shows a "generic PnP monitor" connected.  I'll disconnect the cables, shut down the laptop, wait, power back up, try connecting again, and still I get the 1920 resolution.  "Windows-P" toggle offers no help (unless I'm overlooking something) - it tells me that my max resolution is 1920, doesn't even list 3840.  And if I try to go back to only the laptop display, it will revert back to the LG monitor after 5 seconds (even when I click on "keep these settings").  When I switch to "duplicate" displays, I see the following screens: [IMAGE] [IMAGE] [IMAGE] I see that the "Active Signal Resolution" for the monitor is 1024 - don't know why? Intel(R) UHD Graphics & NVIDIA GeForce GTX 1650 Ti display adapters loaded.  Any ideas?
XPS15 9500 "forgets" 4K monitor