Not that it'll be any comfort to anyone experiencing this problem, but:
1. Ever since upgrading from MacOS 11 (which was a load of rubbish as far as I was concerned)
all 3 of the monitors connected to my 2018 Mac Mini have worked.
1.1. However, one of my monitors connects through an Apple USB-C to VGA converter which also has
a USB pass-through which is connected to a powered USB hub. If that hub is connected when the Mac Mini is booted
up that monitor stays bland: as soon as it is disconnected, the monitor display works, after which the hub can
be attached: very odd indeed.
2. But, oddly enough, 2 days after the release of MacOS 12.3.1 that update is not showing 'over here' in
Bulgaria.