Comment # 4 on bug 969094 from Hrvoje Senjan

Yes, and no. That package is to be used when no DE is otherwise used in order
for Qt applications to pick up users settings.

With current packaging, i see 2 issues:
1) the name is totally wrong, as it indicates it's one of Qt Project's
'products'
2) it effectively overrides any platform plugin (be it Qt's, KDE's or something
else), just by being installed, leading to your sitution.

I'll try to see with libqt5-qtct's maintaner what we can do about that.