SUMMARY

The multi-monitor layout is different each login on Wayland.  This affects a number of related settings: position, size (zoom), desktop background, panel configuration etc.

The cause appears to be the name assignment in `~/.config/plasmashellrc`.

First login:
[ScreenConnectors]
0=eDP-1
1=DP-8
2=DP-9

Second login:
[ScreenConnectors]
0=eDP-1
1=DP-6
2=DP-7

On my system, the laptop screen (eDP-1) is always screen 0.  The other displays are the same physical displays on the same physical ports, but the DP-n number changes each time.

STEPS TO REPRODUCE
1. Login to Wayland and configure screen layout
2. Logout + reboot
3. Login to Wayland

OBSERVED RESULT
The screen layout has changed

EXPECTED RESULT
The screen layout is the same

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma:
(available in About System)
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.2+kde294

ADDITIONAL INFORMATION
The laptop has hybrid Intel/Nvidia graphics.  The Intel UHD graphics drive the primary laptop display, the external monitors are connected via a Thunderbolt dock, which is routed via the Nvidia chip.

This may be related to bug 427278 - although that was for X11 & VNC, and is resolved.