GtkMediaStream can't seek()

According to the documentation, this should be enough to seek from the current timestamp to five seconds forwards:

def seek_fw(self, widget, _):
    timestamp = self.stream.get_timestamp()
    self.stream.seek(timestamp + 5000)

Which is a big deal, because before the Gtk.Video() widget when using GStreamer directly there was no such luxury as a relative seek.

But it does not seek, it only pauses for half a second and then resumes from the current timestamp. And if I try to seek to a precise timestamp, all I get it the stream resets to timestamp 0.

I tried a lot of things - I’ve been at it for a few day now - of course I tried a lot of video files to make sure the video stream itself wad not causing this, in fact all the streams I create are stream.is_seekable() => True so I really don’t see what is going on here.