I have had ongoing issues with this since I bought my 2018 Mac Mini (Intel) and with every OS since then. There was a set of firmware updates in early 2019, and also in the Big Sur releases that helped, but the issue appears to be back for me (12.1.* and 12.2 public releases). I have 2 X 4K Samsung screens, one directly connected by HDMI and the second connected through an OWC Thunderbolt 3 Dock using USB-C(TB DisplayPort)-to-HDMI. The only thing that seems to work reliably for me is to disconnect the dock monitor, then power on the computer, then reconnect the dock monitor after a successful boot. If I have both connected when the machine is powered on, the internal HDMI fails to work - Every time. I have a Feedback report FB9845580 showing the Console Log errors when I try to connect the internal HDMI. Once this happens, I have to shut down the computer, disconnect the external dock monitor, and even then the internal HDMI port doesn't reliably come up. When that happens, I must do a PRAM reset to restore the internal HDMI. So my suggestion: 1: Disconnect any external dock you have after you power off. Then connect the HDMI and power on. 2: Disconnect external dock/ monitors. Connect the HDMI to internal monitor. Then reset PRAM. Hold CMD-OPT-P-R (all 4 keys) when machine is off. Power on machine. Continue holding all 4 keys until machine auto-reboots. Then release the keys. I found this was required in some of the earlier betas to restore the HDMI. Once it finishes booting, you can reconnect any other external monitors. Also, please put in a Feedback Report so Apple Support gets the detailed log messages. Maybe that can help with finally resolving the issue for us Mini 2018 owners! You should see Kernel messages related to IGFB in the Console log when the issue occurs. Hope this helps.

@meaton Feedback filed with sysdiagnose : FB8892578 This is for poor 1000BT performance. Using an external 1000BT Thunderbolt adapter addressed the problem, so this is definitely an issue with 11.0.1 RC2 and the Mac Mini 2018 built-in ethernet.

Later beta 11.0.1 Beta 5 (?) worked for me earlier this week. I saw expected 200+Mbit throughput on DSLReports.com using Mac mini 2018 + high-speed cable modem. But with the new 11.0.1 RC 2 candidate, performance fails with same problem. I'm seeing that EVERY site / downloader is getting ~20Mbit at best (1/10 performance). This is using Microsoft Update, Safari, FileZilla, etc. I reset PRAM, tried to force the hardware to different 1000TX modes, etc. but still see very poor performance in 1000TX mode. When I reboot the machine back to Catalina with same hardware and performance is back. I'm filing a Feedback on this now.

My install of the Beta (prior to RC 2) on Mac mini 2018 required the firmware update to be run. Update took a LOOONNNGG time, but did update the firmware during the Beta install. Currently reports this version:   System Firmware Version: 1554.50.3.0.0 (iBridge: 18.16.12561.0.0,0) My install was a clean install onto an external drive that was freshly erased before install. So your log in particular looks like the firmware updated didn't run. Was the install onto an external drive? There were issues with firmware updaters years ago (before Sierra?) not running when the OS install target disk was not the internal drive.