Same with me, MacBook Pro 2019 16 inch running Big Sur 11.1. Trying to connect to a projector via the hdmi connector of a usb-c Portside dock and watch a movie with the lid closed using Kodi. Random flickering issue, happening from once every few minutes to several times a minute, unaffected by all the possible settings discussed here. Very disappointing, I guess I'll have to use my vintage MacBook Pro 2010 which does the job fine.. Unfortunately I don't know if I would have had the same issue prior to updating to Big Sur, as the computer is new and I updated during the first days of operation.