Just my two cents, which can hopefully help with reproducing this.
It most consistently occurs for me when the screen automatically locks, but the mouse is moved to stop it fully locking before the “Allow unlocking without password” period of 5 seconds is over. It attempts to show the lock screen before going black. It doesn’t let me unlock with the password, but fingerprint (configured through KDE) works on the black screen, with only the cursor visible.
I am running KDE Plasma 6.0.5 on Manjaro.
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