Same here. Macbook Pro 2017 13'' no-touch-bar model. Before updating to Big Sur, I connected to 2 external HDMI monitors with no problem at all. Just after updated to Big Sur, now one of the external monitor will intermittently flicker (flashing or black for 2-3 seconds then come back), especially after sleeping for a while. This is really annoying and deminishing the usability of Macbook! Please fix this ASAP, Apple! And please give it a thorough test before releasing a new OS!

Just want to let you guys know that I've fixed this! This was my reply on page 2: Same here. Macbook Pro 2017 13'' no-touch-bar model. Before updating to Big Sur, I connected to 2 external HDMI monitors with no problem at all. Just after updated to Big Sur, now one of the external monitor will intermittently flicker (flashing or black for 2-3 seconds then come back), especially after sleeping for a while.  This is really annoying and deminishing the usability of Macbook! Please fix this ASAP, Apple! And please give it a thorough test before releasing a new OS! - Then I re-installed Catalina. - The problem was still there! BUT it was much better than on Big Sur! Less frequency and less black screen time! Then I replaced my ThunderBolt adapter HyperDrive NET 6-in-2 Hub to the apple.com suggested CalDigit Thunderbolt 3 Mini Dock -Portable. The problem is gone! Then I upgraded to Big Sure again. The problem is gone! So my conclusion is it might not be Big Sur's problem, but Big Sur might maginfied the problem.