problem extracting frames from 480p Youtube videos using DirectX Samplegrabber

I thought so too but it's only partially true.
Seeking backwards messes the callback function, yes. But any kind of seeking messes also the time
of the captured frame.
Take a look at the "Type: Video frame index ..." on each frame. It should be ascending from top
to bottom:

Is just the order wrong, or do the timestamps also not match the frames?

The frames are actually in the right order, just their timestamps are wrong.