I have this issue as well. I have 2 external displays, both LG high-end IPS, high-refresh displays. They were working fine yesterday. Came into my office this morning, and now only my 27" monitor in portrait mode will be detected. Absolutely 0 changes to the configuration from the day before. I've They're both connected to their own dedicated USB-C port on the MacBook Pro. I've tried restarting several times, I've tried resetting SMC and PRAM and the usual swapping cables around with the display that IS detected, nothing works. The monitor still works fine and is detected by my PC without issue. I'm sort of done with Apple at this point. I've had SO MANY issues getting all my devices to consistently work on my 2019 MacBook Pro. I'm just over it, the old adage "it just works" is no longer true with Apple computers which was the whole reason we used them. If I wanted to debug issues every day I'll just use Windows.