First there's an error from KWin at the very top of the screen, then once the splash goes away, you get the Compositor system settings module in the middle of the screen. Is this really necessary?
i don't understand your bug at all.
The first login with a fresh profile results in a KWin error at the top of the screen while the splash is going and then the Compositor system settings module automatically launches, just like MageiaWelcome.
Cannot reproduce in current Cauldron/Plasma.
Problem has reappeared.
This post describes the error message from KWin that appears during the splash:
is it still valid with plasma 5.8.6 ?
I only ever saw this problem on a physical machine at my old job. I never had this problem in a VM, so I can't confirm whether it's still an issue. I'll close this as OLD. If anyone else sees this on real hardware, they can reopen. Sorry for the delay.