The user list of layouts set in the “regional settings” are always reset to default after OS reboot, maybe after disconnection/re-login too.
I have set fcitx 5 Wayland in “virtual keyboard” pane in “input peripherals” tab in kde settings, then in “regional settings” the user list of layout in the “input method” pane.
Can you please share your system configuration (operating system, Plasma version, etc.)? The easiest way is via the Info Center app, using the Copy Details button:
Also, would you be able to share screenshots of where you’re changing the settings that are then getting lost? I’m asking as the names of System Settings tabs/sections that you quoted are subtly different from the ones I see on my device, and I’m wondering if you might be on a different/older version of some components?