Just for those that are having issues with HDMI - HDMI video connections after upgrading to Catalina, eventually managed to get beyond basic support level and spoke to someone who appeared way more knowledgeable.



Many are having issues getting a basic HDMI connection to work at all or only through weird and wonderful workarounds. Apple has confirmed to me they are aware of the issue and they advised it was an issue they could replicate when using HDMI -> HDMI.



Of course, no information as to when this may get fixed.



For most, the answer is to use USB-C to HDMI or DP but it is not assured to work but seems to in many cases.



For those that are going to revert back to Mojave as a solution, a number of people are noting that this does not fix the issue (it didn't for me), indicating there is perhaps some sort of firmware update taking place with Catalina that will not be rolled back when you install Mojave.



The most affected users from what I have seen are those using 4k displays, it makes no difference what cable is used they can only get back up and running by connecting to a lower resolution monitor.



So just a warning really, add any updates if you have them but it is clearly is an issue and I suspect won't get resolved swiftly.