Bernie has noticed that when he puts his M1 Mac Mini into portrait mode, that his monitors have problems. Leo says that the M1 is a new system and as such, it's bound to have some limitations until there's an update, especially with drivers. Leo thinks that something has changed with macOS Big Sur, and as such, Bernie's monitors can't handle the new configuration due to having an outdated driver. So he advises contacting the monitor manufacturer and see if there's a new driver for Big Sur. That's likely where the issue is. He may also want to check the resolution in the system monitor settings.
From the chatroom ... SwitchResX will provide a lot more resolution and orientation options. Try that.