Comment 2 for bug 1452313

Confirmed.

Particularly, in my case notifications about Wi-Fi don't fade away (losing connection, succesful connection, 'can't scan while already scanning')

Temporary solution - restart plasma:
Alt+F2
killall plasmashell
plasmashell

Then the notifications 'catch up', i.e. the ones that should have appeared after the one which was stuck, appear for a brief moment and the fade away normally