Mac mini 2018 hdmi white noise
There are lots of reports of monitor problems with the 2018 Mini: so many that it seems unlikely to be anything other than a problem with the hardware itself (or specific firmware/software to that model).
I myself see the same problem: white noise on the HDMI-connected monitor after Sleep.
Using 2 TB/USB-c ports rather than one TB/USB-C and one HDMI may be a factor, rather than the cable itself.
It's unknown at this stage whether Apple may be able to fix it (with enough bug reports) in a software update, or whether it's a permanent hardware issue.