If I set some splash screen and set some applications with graphical interfaces to autostart (systemsettings5 -> Startup and Shutdown -> Autostart -> Add program), whenever KDE is loading, these applications interrupt the splash screen - i.e. before the desktop environment has terminated loading and while the splash screen is showing, the applications from autostart show up first for a short while, before the desktop environment is fully loaded, and then the splash screen returns until it finishes loading.
Yes, I can confirm such behavior when I set yakuake 3.0.5 to autostart. SOFTWARE/OS VERSIONS Operating System: Arch Linux KDE Plasma Version: 5.15.2 KDE Frameworks Version: 5.55.0 Qt Version: 5.12.1
This should be long since fixed with the systemd-enabled Plasmashell startup process.