I'm afraid, the problem can be in some related QT or KDE library: I've tried to downgrade sddm and sddm-theme-breeze to the versions from zesty (0.13.0-1ubuntu5 and 4:5.9.4-0ubuntu1 correspondently), but the problem is still there.
KDE version 4:5.10.5-0ubuntu1, QT version 5.9.1+dfsg-5ubuntu2.
I'm afraid, the problem can be in some related QT or KDE library: I've tried to downgrade sddm and sddm-theme-breeze to the versions from zesty (0.13.0-1ubuntu5 and 4:5.9.4-0ubuntu1 correspondently), but the problem is still there.
KDE version 4:5.10.5-0ubuntu1, QT version 5.9.1+dfsg- 5ubuntu2.
PS The user icons are also invisible.