see the screencast here https://bugs.kde.org/attachment.cgi?id=169439 launching the "add widget mode" opens a toolbar sliding in from the left, which the "edit mode toolbar" doesnt seem to expect. So the "edit mode toolbar" is overlapped by the "add widget toolbar". there is some coordination needed.
This is fixed by the new Edit Mode that will be landing in Plasma 6.1. Testing this on the 6.1 Beta (6.0.90) doesn't show the issue anymore.