I've encountered this before (somewhere around 4.7), opened bug 281618, it got fixed ... and it seems to be back. When I restart a video, it has lost track of its current position. I'm using KDE 15.08, but with 4.14 it was fine. I may need to downgrade dragon for this.
Please change your phonon backend to the vlc one and check if that fixes it. I am pretty sure this is a phonon backend problem (still).
"Phonon VLC" is the only backend that shows up in my KDE system settings, so I'm pretty sure that's the one I'm using :) I don't even have any other backend installed.
Ah, try with gstreamer then please.
My apologies. After a debacle involving nvidia-drivers-368.16 forcing a downgrade and reboot, this problem has gone away (still using vlc, mind you). I don't know what caused it originally, but it has disappeared. There are other problems, which deserve separate reports, but this one seems gone.