I tried using Users in KDE Plasma 5 System Settings. If I choose a user avatar from the gallery, the user avatar is not updated in kickoff, screen locker and login manager (sddm). A similar bug is described here https://bugs.kde.org/show_bug.cgi?id=422177. But I didn't understand if the problem was solved. At least in FreeBSD, it doesn't work.
FreeBSD 12.2-RELEASE-p10 amd64, KDE Plasma 5.22.5, KDE Frameworks 5.86, Qt 5.15.2
I found a workaround. You need to make a symbolic link in user's home directory.
ln -fs /var/db/AccountsService/icons/$LOGNAME .face.icon
Can you describe in detail how you change the user avatar? On my system there is no /var/db/AccountsService (or /var/lib/AccountsService, which seems to be the Linux path for the same).
The "Users" module in System Settings does not seem to be the right one (it shows me an empty list and a "John Doe" user)
I don't know how /var/db/AccountsService/icons/ and /var/db/AccountsService/users/ directories were created in my system. I did not create directories myself. I installed the KDE5 a long time ago and am constantly updating it.
I have two accounts in the system: root and User. The "Users" module in System Settings shows me only "User" user. Thus, my list is not empty. If I log in with an root's account (I have it allowed in SDDM), then I can change the avatar in "Users" module in System Settings. Also I can change the avatar by logging in with account User. But this still requires you to know the root's password. Thus, in order to change the avatar, you need to know the root's password.