I'm so glad to have found this thread !
I have the same issue, and in fact with the exact same configuration BlkPengu has: a MacBook Pro 16" (2019) and an LG 34" widescreen. (A great combination, BTW.)
I deliver a lot of presentations and courses, connecting various projectors and screens to my MacBook. And back at home, not always but e.g. once in 3 to 4 times, the system still thinks the earlier display is still connected. My LG is also detected but at the resolution of the former !
The SwitchResX tool helped me discover that in such an instance, there are in fact THREE displays (falsely) being detected: the MacBook's built-in, an earlier connected display like a projector or presentation screen, and the physically connected LG, showing up as a 'ghost' display. So the system is strangely still aware of the recently connected projector with a certain resolution, while the LG is also detected but getting the signal and resolution of the assumed other external display.
Using the plain Display Preference panel (or with special option-invoked settings) won't work. Even the most drastic Rotate setting doesn't do the trick. The only solution is to completely Restart.
Below two samples of a settings panel in SwitchResX:
I expected that it might be possible to 'kick' the system's display sensing routine, but SwitchResX's developer Stépahne Madrau investigated the issue last week, and said this isn't something a user can fix with available tools or user-friendly hacks. It's definitely a system bug or flaw, which needs to be addressed by a system update.
With this information, I'm going to ask Apple what I need to do with this.
I'll notify you all ASAP when I get some response.