SUMMARY
I have a laptop with a 16:10 aspect screen internal screen, This machine is used with an external 16:9 screen as its primary screen. Every time I boot up it connects to the monitor with a 16:10 resolution despite the settings being set for 1920x1080 (a 16:9 aspect). In addition to this the list of supported resolutions only shows up to the  highest 16:10 resolution the monitor supports (1650x1080 in my case). Making it impossible to select the correct resolution as not show in the list! In order to see the real list of resolutions I need to unplug the monitor with KDE running and plug it back in.

 This could be related to https://bugs.kde.org/show_bug.cgi?id=469113. 

STEPS TO REPRODUCE
1. Use a laptop with 16:10 internal screen.
2. Plugin the monitor, turn on the laptop
3. Set the correct settings for your external monitor. The external should be in a different aspect ratio then the internal aspect ratio. Disable the internal screen.  Be sure you have save the config  for "this specific arrangement".
4. Reboot 
5. Upon reboot check the resolution of the external screen

OBSERVED RESULT

The Saved resolution for the external screen (1920x1080) is not used and instead a resolution matching the internal aspect is picked. This list of available resolutions in the display KCM now stops at the highest resolution that the monitor supports in the aspect ratio of the internal screen.

EXPECTED RESULT
The previously selected resolution is selected for the external monitor and the full list is shown in the Display KCM for the external devices supported resolutions. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version:  5.27.6
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8

Additional Information
Laptop Screen Default Resolution: 1920x1200@60hz
External Screen Default Resolution: 1920x1080@75hz
HDMI is used for the connection.
Tested on Wayland and X11
Tested with the laptop open and closed while booting.
During boot the external says its mode is 1920x1080.
If you correct the resolution then log out . The correction will persist until reboot