So, it looks it is not an installation issue, but screen resolution after install, thus probably a graphics driver missing/not detected correctly.
Changing tags, and is it possible to post some logs, to figure the bug? Octopi > Tools > SysInfo would be helpful, and possibly /var/log/Xorg.xx.log
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EDIT:
Seems a well known issue since Plasma 5.15, solution posted:
https://forum.kde.org/viewtopic.php?t=157213
Systemsettings > Startup and Shutdown > Background Services > untick KScreen 2