I had this issue as well, but resolved it as follows: My Mac mini uses USB-C to connect a device with 2 HDMI ports. Each of these connect to a monitor. I tried everything I read in this article and the comments, to no avail.
What worked for me was swapping out the cable positions on the device. Left>right, right>left. Worked perfectly and my Mac mini recognized both of my monitors.
Hope this helps someone.
spmoran
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