Comment # 2 on bug 959849 from Wolfgang Bauer

As you are already using Plasma 5.5.1 from an additional repo, could you please
try with the latest Dolphin 15.12.0 from KDE:Applications repo?

I cannot reproduce this problem here (13.2, Plasma 5.5.1, Frameworks 5.17.0,
KDE Applications 15.12.0).

Although such a problem (application not quitting when closed, caused by a
bugfix in KF 5.17) is only known so far for Kate and Ark (and I've seen it with
Konqueror5 too but that hasn't been released yet... ;-) ), where it has been
fixed in 15.12.0.