[Dumyat]

Is the wrong duration always 1 hour, regardless of the correct value?

I'm asking because there is a place in the code that (for whatever reason) sets duration to 3600 secs if end time is less or equal to start time...

No, unfortunately not. I have seen various discrepancies, ie a 2hr programme showing as 1hr or 3Hr. Or maybe even 1hr 30min. But as indicated on the screen capture above the nearer you get to the broadcast time, it seems to correct itself. So quite odd and often difficult to spot.
First noticed the issue in December and seems to be quite random.