One possibility is to search through the Siduction forums for recent posts. Several there have been running KDE on sid for many years. Check the Debian BTS. Maybe also try on irc. Since it does not happen in pcmanfmqt based on the same QT5 libraries as KDE - it probably is in some of the tier 2, 3, or 4 KF5 code. The branch of KDE you are runnng is destined for LTS and what with the approaching freeze, squashing of all bugs like this is job #1 for most Debian KDE devs. Short answer is, it is probably going to get fixed in the near future.
One of the shortcomings of Sparky is the inability to easily foward bugs up to Debian, it is not anyones fault, it just is. We do have the ability to search the wide world of Debian for answers. As I said, "it just is."
One possible solution for those of us running multiple machines is to have one of your machines run Debian testing. It is the same debian packages for KDE. That way one can send bug reports up, and then also mention that it affects 2 other machines running Sparky. You advance the cause of Debian and Sparky gets a plug. These views are entirely my own and not recommendations, just options to think about.