Ok, thanks for mentioning the 4K screen and I would for sure try the nvidia drivers with that for better performance (you can check both arch and ubuntu forums for results).
Nouveau does work for some, but, has flaws, this looks like one of them, but, only you can find out by trying both nvidia and nouveau drivers and seeing for yourself what works and what don't.
You probably also have stock sddm configs which is why mine are different, usually with sddm it will give you a "white" sddm screen (latest upgrade or upgrade before last, I can't remember now), I also have a "timer & blur" tweak in mine, you can find out more on other tweaks at
kde forumsUsually when the login screen is messed up after a full-upgrade editing the LockScreenUiqml usually fixes it (99% of the time)
My login screen (needs work yet)