The GUI keeps asking me to apply the wallpaper and after logging out, it doesn’t work 
I have to change the plasmalogin.conf to change the wallpaper manually, what could be the issue?
Here are the logs:
You need to do a second step.
For plasma-login manager:
In System Settings go to the Login Screen.
Click the Apply Plasma Settings... button at the top of window and click Apply.
DId just that, tried to set the wallpaper to the Oxygen one and it set it to the default one 
I tried to reproduce this on Fedora 44.
What is your distro?
To set the wallpaper I use the System Settings/Login Screen and clicked on the Configure Appearance... button.
I can set a new wallpaper, I choose one of the stock ones Fallen Leaf, hit apply and logged out. The wallpaper showed up.
Then I set it back to my personal wall paper and that showed up in plasma login screen as well.
Are you using wall paper you provided?
I recall I had to do something to allow my personal wall paper to work.
Then it sounds like it’s a bug in EndeavourOS, you will need them to fix the packaging.
tried to reinstall the packages but it did not work sadly :<
I am having the same issue here on CachyOS, additionally my keyboard layout on KDE Plasma Login is not my systetem default but QWERTY until I am logged in and my default QWERTZ works. On any TTY my QWERTZ is applied correctly.
i dont have that issue anymore when i installed cachy which is very weird, that endeavour install was brand new.
I am also having this issue on CachyOS, plus am unable to set avatars using the plasma gui. As you noted, I can of course change it manually through file manipulation. My CachyOS install is perhaps 2~ weeks old, kept up to date daily, and is mirrored across both a laptop and a dekstop. And a friends desktop is displaying the same issue. I am also very sure it worked properly about a week ago, as I had a “placeholder” image for both the login background, and the user avatar, that I finally got around to updating.
It is a little interesting, that after this mornings update the config file pointing kde to the avatar in question is no longer in accountservice, but has been moved back to the .face scheme.
This is happening to me as well on CachyOS!
With some help in the cachyos discord, some folks told me to check the /etc/plasmalogin.conf file and I discovered that after applying the wallpaper, the plasmalogin.conf updates with a path to the file in /var/lib/plasmalogin/wallpapers/ but the wallpaper itself doesn’t get moved there so it’s pointing to nothing!
Uhoh, seems we’re hitting some variant of /var/lib/plasmalogin breakage like
But here it’s for the wallpaper…