I meet the same flickering issue on a brand new Macbook Pro 16 inch 2019, 2.6Ghz 6 core Intel Core i7, running Big Sur 11.5.2. Once connected to my Dell U2518D external display, it flickers every 6 seconds, approximately. I do not meet this problem with the old Macbook Pro 15 inch 2017, 2.8GHz Quad-core Intel Core i7 that I own, that runs Catalina 10.15.7. As I work on an external display the whole day, for computer graphics, this is a severe trouble.