I have two 4K monitors that worked seamless prior to Big Sur. One has stopped working altogether with no apparent settings to reinstate the monitor. I have the highest end Macbook Pro on the market, so graphics card isn't the issue. I'm on MacBook Pro 16-inch 2009, 2.4 GHz intel 8 core i9 with 64gb 2667 MHz DDR4 ram and AMD Radeon Pro 5600M 8 GB graphics card with 8 TB SSD hard drive. Which is the top configuration. There are four Thunderbolt3 ports, designed for 4K power. I am using two for 4K monitors, 1 for hub and 1 for external hard drive. It seems to me this is a driver issue around batter power regulation that is defaulting to turn off the additional 4K monitor. I can't find a solution to this online yet and this has interrupted my workflows. Very annoying since I paid so much for the highest spec machine.