I contacted Apple about the HDMI – HDMI video connection issues after upgrading to Catalina, and got no info about when this could be fixed. Reverting back to Mojave did not fix it. Can someone inform me if any updates on the issue are available?
The cause of that is in the following link with the possible solution
If 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 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. But give it a try.
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, 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 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.