I received my TBT3-UDZ just on Friday here in the UK. Sadly, I am also having problems with running 2x monitors on HDMI.
My current computer is a MacBook Pro 16" from late 2019. My monitors are Dell U2718Q.
I have been using these monitors for over 2 years now with my old Thunderbolt 2 setup with my previous Mac and since getting the new one, I have been using a TB3 to TB2 adapter. I had two TB2 docks daisy-chained with each one driving a monitor.
What I have noticed is that it does seem to work intermittently, when the lid of the MacBook is open. However, I use it in clamshell mode and soon it will be sat in a vertical dock. What I am finding is that one of the monitors drops right down in resolution. The highest option is 2048x1080. Everything looks completely zoomed and blurry. The other monitor works fine.
If I boot into Windows using Bootcamp, both monitors work fine. No problems. Maybe suggests a bug in macOS perhaps? I am running macOS 11.1 Public Beta 2.
I have tried resetting the SMC and the NVRAM/PRAM on the Mac, but it hasn’t made a difference.
Wondering whether to purchase some DisplayPort cables and try those instead but these HDMI cables are good ones and have been fine for the past 2 years.