macOS Catalina BUG: Brightness jumps to 100% when connected to an external monitor
Under macOS Catalina there is currently the problem that the screen brightness automatically jumps to 100% when an external monitor is connected. This problem is caused by a bug in the current macOS Catalina version, 10.15.3.
Please update macOS Catalina to the latest Catalina version or update to a newer macOS. This will fix this issue.