Mac Mini thinks there is a phantom third display

I have a 2018 Mac Mini running macOS 10.15.7 Catalina. I have two displays attached. One display was attached using a HDMI to HDMI cable. Today I sat down at my mac only to find that that display was dark.

After some fooling around and seeing that other people have had problems with the HDMI port on the 2018 Mac Mini, I switched to a new cable using USB-c (plugged into a Thunderbolt port on the Mini) to HDMI (on the monitor side). Now both monitors are working fine. The strange thing is that the Mini thinks there are three displays as shown below! I'm guessing that the Mini thinks there is still a display hooked up to the HDMI port even though there is no cable attached there. Rebooting does not do anything. Anyone have any idea as to how to get rid of this?

In case it matters, the other display is also using a thunderbolt port.