Just to add my experience. Just upgraded from Fedora 33 to Fedora 34, and I was having this issue on F33 and _still_ have it on F34, where I am using Plasma on X11 login (not wayland), using the nvidia driver.

My setup is a Thinkpad P51, With an external monitor on HDMI, and a USB-C docking station with another (Display Port) Monitor attached. I am usually using all three monitors at the same time. I am typically loosing my desktop configuration as described above after suspend/resume (as observed on F34), but had the other occasions (reboot, ...) on F33 also.

So, this is not fixed for me. And I can remember this issue for ages. You almost get used to it until it annois the heck out of you

I continue to have this problem too.

I would just like to comment that behind this problem is an inherent problem with the design. These "Plasma Profiles" (as per hellblade's definition above) are a hidden construct not readily visible, inspectable, or controllable by the user. They are apparently just supposed to work "automagically." This seems to me utterly counter to the Linux philosophy of giving control to the user, and more specifically the KDE philosophy. This is a typical Windows approach to things, not a Linux approach.

From a pragmatic standpoint, such automagic approaches are bad enough when everything works correctly (bad because they depend upon assumptions about what the user wants and put these behaviors out of the user's control), but when everything DOESN'T work correctly, which has been the case with multiple monitors in KDE Plasma for years upon years and probably for all time, then the situation is the worst of all worlds.

Same issue here.
I run docked with 2 external displays and the laptop screen off. I set up wallpaper and a panel for every display.
When I undock, layout for the laptop-only screen-layout is fine. When I redock, the two external displays shows nothing but a black wallpaper, and if I disable the laptop display, I have no panel anymore. Seems like it completely forgot about my initial 2-screen layout (panels, wallpaper, dpi-scaling).

Operating System: Manjaro Linux
KDE Plasma Version: 5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
Kernel Version: 5.15.74-3-MANJARO (64-bit)
Graphics Platform: X11
Processors: 12 × Intel® Core™ i7-9750H CPU @ 2.60GHz
Memory: 15,3 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 1650/PCIe/SSE2
Manufacturer: Dell Inc.
Product Name: XPS 15 7590