After "Suggest this crash is related" button pressing, dialog appears. Here you can select to assosciate bugs. But this option does not take effect. See video: https://vid.me/sfvz Reproducible: Always
I get the same with KDE Frameworks 5.16.0 and Qt 5.5.1 on an openSuSE Leap 42.1 x86_64 box.
Bug still in recent KDE Frameworks 18.0 + KDE Applications 15.12.1 + KDE Plasma 5.5.3
*** This bug has been marked as a duplicate of bug 355807 ***