When the mouse is moved to activate the controls on the lock screen, instead of blurring, the background fades to black.
It appears this only happens with dark backgrounds, because the blur is very strong and has a grey overlay.
Hmm, maybe we should tone down the darkness a bit here. Something to investigate for 5.14 after I finish up my Login screen tweaks.
Can you please test with master (KDE Neon Dev Unstable)? The blur and darkening have been reduced so the problem might have been solved.
Yep, this was effectively fixed with https://cgit.kde.org/plasma-workspace.git/commit/?id=2eec8cb1b5056cac670c2a29b718b250477eab2f
*** Bug 408503 has been marked as a duplicate of this bug. ***