Not possible to rotate monitor on dual screen setup - Screens overlap
On a dual monitor setup it is not possible to rotate any of the screens into portrait mode (Landscape and Ladscape flipped work). When selecting a monitor to switch to portrait mode, it extends the rotated monitor about halfway into the second screen and overlap. This overlap is also visible in the display settings. It is not possible to rotate the monitor, no matter what one tries.
A temporary workaround is to set the rotation with xrandr or with the NVIDIA X Server. However, once the machine is rebooted or the screen lock turned on, the settings are "forgotten" as if one made no changes to the screen orientation at all, back to zero.
Here is also a thread on reddit with more people experiencing the exact same issue: https://www.reddit.com/r/Ubuntu/comments/g82u58/cant_rotate_monitor_orientation_in_ubuntu_2004/
And the filled bug report on launchpad: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1874217
About my system:
OS: Ubuntu 20.04 LTS x86_64 (all up to date)
Kernel: 5.4.0-26-generic
Uptime: 16 mins
Packages: 1575 (dpkg), 9 (snap)
Shell: bash 5.0.16
Resolution: 2160x3840, 3840x2160
DE: GNOME
WM: Mutter
WM Theme: Adwaita
Theme: Yaru-dark [GTK2/3]
Icons: Yaru [GTK2/3]
Terminal: gnome-terminal
CPU: Intel i7-6900K (16) @ 3.200GHz
GPU: NVIDIA GeForce GTX 1080 Ti
Memory: 2385MiB / 128743MiB
If needed, I can provide more information.