The solution is removing the
DrawBackgroundPerScreen_*=true
lines in [Background Common].
[Desktop0Screen0] does it's share to it as well, but deleting the lines
noted above seems to be enough. I assume [Desktop0Screen0] contains the
settings for "overdrawing" the regular screen and does nothing if the line
above hasn't been specified.
Greetings,
Sandro
This bug has been filed at
https://bugs.kde.org/show_bug.cgi?id=133378