Can you try the other splash screen types? I was thinking Minimalistic or Simple - to see if they show the issue as well.
Based on the Fedora splash working - I suspect they won't show the issue.

If this is the case, then it is possibly a bug in the splash screen handler which those splash screens are using..

That's what it seems to be.

Please file a bug report at bugs.kde.org, after determining which KSplash backend is used by the splash screen(s) in question.
This should be visible in the command which is run to display the splash screen - which you can monitor on another tty during KDE Login.

How exaclty? Never had to start ksplash manually before.

You should first switch to a seperate virtual terminal (VT) through Ctrl + Alt + Fx (where Fx is F1, F2, etc) and login there.
Then enter the command "ps aux | grep ksplash" but do not execute it yet.

Then transfer back to the login screen (usually virtual terminal #7 - ie. Ctrl + Alt + F7) and login as you normally would. Once you have commenced the login quickly transfer back to the virtual terminal you were using previously and execute the command you previously entered (which is above).

You should see a "ksplashx" or similarly named process in that output - copy that down and post it here.