Well, then as I thought, you had not updated your system anymore since we were still on Plasma 6.3. We went from there to Plasma 6.5, but as of Plasma 6.4, KDE decided to split off the X11 support in Plasma and make it optional, because as of Plasma 6.8, X11 support in Plasma will be completely discontinued.
Now, if I were you — I do not know whether you’ve got an Nvidia GPU — I’d try switching to Wayland at the login screen, given that X11 support is going to be dropped.
But your system was still on X11, and due to this having been split off, you could not log in again without the proper packages. Installing plasma-x11-session fixed that — again, for now, because Plasma users running on X11 are on borrowed time.
I’ve already explained it to you, in post #11.
MS-Windows is another beast altogether. For one, it is a point-release operating system, whereby an update only brings along only small patches which do not radically change any functionality. There are several GNU/Linux distributions like that as well.
In a curated rolling-release system like Manjaro, a bundled update means that there may be significant changes to the way core components of the system work together. In your case, there was a significant change in the way KDE Plasma works, and a move from upstream to Wayland, with X11 only becoming a still temporarily supported non-default option.
This then, compells me to post the following short essay, and please do read it, because it would appear that this applies to you. 