Please play this video to understand a complete reproduction of problem: https://youtu.be/lgn0wkwWISA
Which Plasma version? Use kinfocenter to find out.
I see this behaviour on current latest Arch Linux on my laptop with Plasma 5.10.4, KDE Frameworks 5.36.0, Qt 5.9.1.
And on my PC, where working KDE Neon project with Plasma 5.10.4, KDE Frameworks 5.36.0, Qt 5.9.1.
Looks like duplicate of https://bugs.kde.org/show_bug.cgi?id=377956
*** This bug has been marked as a duplicate of bug 381242 ***