It's almost as if the signal for the monitor to turn on is not being sent. MacOS appears to recognize the monitor and resolution, as it is shown in display settings, and Apple Support was able to remove into my computer and see both screens. Secondary monitor (same model) connected via HDMI->DisplayPort adapter (BENFEI), then DisplayPort->USB-C cable no longer works after updating (worked fine on 11.1 before updating). Primary monitor LG Ultrafine 4k (Mac version) connected via Thunderbolt. Upgrading to 11.2 broke my second external monitor connection. We saw this issue firsthand after upgrading a MacBook Pro to macOS Big Sur 11.2, with the update preventing an external monitor from connecting with USB-C, and there are several other complaints from affected users.

Apple support had no fix, so I guess I'll wait on 11.3 to fix the 11.2 update which was supposed to fix external monitor issues. I have lost my second monitor and can only use HDMI now.

Warning - Big Sur 11.2 update is breaking USB-C ports for many.

Mac not detecting usb keynote remote