Probably just need to toggle the autologin option in Plasma settings. I guess manually changing the group confused it.
I also have the autologin group on this machine, by the way.
I have done that, autologin is broken I see the setting change in /etc/sddm.conf.d/kde_settings.conf when I toggle and apply in KDE Settings but it is now broken. I can’t have autologin to work anymore, with or without being in the autologin group (I was in this group initially).
//EDIT: OK I’m onto something, I can autologin with Wayland session , I think the value it writes for the Plasma X11 (literally Plasma (X11)) session is just invalid. Will edit when I have the final solution to this issue.
I’m wondering if man initgroups may be of any use; I’ve not done a great deal in this area myself, so can’t say for sure. It’s odd that (re)adding to the group wouldn’t work? 
I will need to get my head down for a bit, if I think of anything later I’ll let you know.
OK I fixed it, it was indeed writing a wrong value. From the Wayland session, I switched the setting to “Plasma (X11)” but it didn’t write the value Plasma (X11) in the file /etc/sddm.conf.d/kde_settings.conf this time, but simply plasma.
Now it writes the proper value plasma instead of Plasma (X11) from the X11 session too. Probably a weird edge case bug or whatever, I don’t get it but it fixed itself by forcing autologin with Wayland, and back to X11, with reboots in between. I tried with and without autologin group for my user, it works in both case so this group is not needed for KDE, only the SDDM configuration. I have no trace of LightDM in my pacman history file, so I don’t know where this group comes from but it is not needed apparently.
I don’t understand the link between me removing the autologin group, and the inability to autologin afterwards, and the way it fixed itself after switching to autologin for Wayland instead of X11 (and back to autologin on X11).
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My guess is by doing this, the file was recreated or reconfigured.
The only thing that changed during all my tests and reboots, was the line Session=plasma in the configration file /etc/sddm.conf.d/kde_settings.conf, initially it was writing Session=Plasma (X11) when I was enabling autologin from KDE Settings (and when I saw that I thought this could not be right, a space in a setting, and parenthesis???!! this is when I became suspicious the value was probably wrong and when I looked at my old config files from TimeShift), but after I enabled the Wayland session, it wrote Session=plasmawayland, and since that moment after the reboot for test, it was writing Session=plasma on subsequent reboots and enabling/disabling autologin, from the X11 session.
I like that solution. 