No idea where to report this but this is the list I keep. Some already have reports I could find as far back as 2012.
default screenshot tool does not copy to clipboard (as soon as app closes(which is immediately after the screenshot is taken) the clipboard data is lost)
fix is to keep the app open until after pasting which adds just so much overhead for no reason
display scaling does not work (125% causes all fonts to be fuzzy)
alt tabbing does not work as expected (shift-tab should go back in the alt-tab menu but it does not (apparently only with SOME keyboars??? since 2012: https://bugs.kde.org/show_bug.cgi?id=294249))
while alt tabbing: every other screen disappears which makes me motion sick
while alt tabbing: clicking the screen you want to go to cancels the alt-tab and you land back to where you started ??????????????????????????????????????????????????? This one makes me unreasonably angry every time it happens.
almost every theme breaks krunner (background somehow shifts when hiding??)
default settings app is junkware
side bar behaviour is terrible unless you have it wide enough for it to go double sidebar (eg: when on home in sidebar: search > click on group > click on back > click remove search > puts you in the group again?????)
crashes when changing mundane settings
every single screen looks different, no consistency across the app
it is using some custom text selection logic or SOMETHING because double click text selection in text fields does not select per word
mouse up outside popup while selecting text closes the popup?? (mouse up next to a button that you mouse downed on correctly cancels the mouse click without closing the popup)
many apps show the wayland icon in task bar (every bugreport to kde is just replied with "yeah that's the apps fault" but it works in gnome)