I'm using an up-to-date installation of Fedora 26: plasma-workspace-5.10.5-3.fc26.x86_64 kde-connect-1.0.3-2.fc26.x86_64 kf5-knotifications-5.36.0-1.fc26.x86_64 libnotify-0.7.7-2.fc26.x86_64 I noticed that at no particular point in the day, notifications stop appearing as translucent, dark gray popups in the bottom right-hand corner, and begin appearing top center, in a large popup with a light gray background, and a heavy black border. My Googling leads me to believe the big popup is a KNotify notification, while the little one is a plasmoid notification. It wouldn't bother me, except you can press the "Reply" button in the plasmoid notification and you will get a reply dialog. Pressing "Reply" in the KNotify window just cancels the notification. Also, the KNotify window's title is overlayed on top of the notification text. No error messages pop up on my screen. I read that errors should appear in .xsession-errors, except (1) that file is months old, and (2) I believe KDE now uses Wayland, and (3) I have no idea where to find the error log for the Wayland session to include in this report ("find . -name wayland" in my home directory produces no results). I realize this is precious little to go on. I will be happy to add more clues if I know what to look for.
The popup from the top is indeed from KNotifications. Seems like a bug in Plasma if it stops handling the notifications. KNotifications only displays its notification popup if no-one else can do it. Re-assigning to Plasma. If clicking on the link on KNotifications doesn't trigger the action, that's likely a bug on the KNotifications side.
Thanks for the report Bucky. I've done some testing on Plasma from git master and I cannot reproduce this issue. Can you please test and confirm if this issue is still occurring or if this bug report can be marked as resolved. I'm setting status to "needsinfo" pending your response, please change back to "reported" when you respond, thanks.
This went away by itself after a short period of time -- maybe a few weeks? It's been fixed, I think, for a couple years.