Since 5.4.95, ksplash screen is in 4:3 format, resulting in a vertical black border on both sides. Archlinux early testers seem to have spotted the cause (https://bbs.archlinux.org/viewtopic.php?pid=1580095#p1580095), but I don't think it's been properly reported here. Reproducible: Always
I can confirm that this behaviour also happens on my machine. (Arch Linux using the KDE beta release).
I can confirm that this also happens on an openSUSE Tumbleweed system. It seems to be introduced with the latest Plasma update.
Just upgraded to 5.5.0, and the bug is solved :)
(In reply to Guillaume DE BURE from comment #3) > Just upgraded to 5.5.0, and the bug is solved :) Same here. I believe that this ticket can be closed.