display – Second screen goes dark

I have a Mac Mini 2018. I have a main monitor which is connected using the HDMI output, and a second monitor which uses a Thunderbolt port with a Thunderbolt-to-HDMI adapter.

After working just fine for a couple of years, the second monitor started going dark, randomly, two or three times a day. Unplugging and re-plugging it solves the problem.

This does not appear to be a hardware problem. It happens with different sets of monitors and cables. I have sets of monitors and cables both at home and at work, and it happens in both places.

When the screen goes dark the mouse is still visible and I can move the white-outlined pointer around the dark screen.

This may have started happening after I did a recent Catalina upgrade, but I’m not sure. I’m currently running Catalina 10.15.7.

What could this be?