I don't think Talefish is comparable in this regard, because gPodder is a lot more complex. From a quick glance, I gathered it passes the value through signals and to python, where it's stored somewhere – didn't have the time or python-fu to check further. Talefish just sets it in a QML Object ("Appstate"), which is auto-saved to LocalStorage once in a while or on app exit.
Assuming the correct values are saved (which they should as long as some kind of progress output behaves correctly in the gui), perhaps the initially scanned duration of the file is wrong and so gpodder thinks there's no way to seek that far into a file? (Please note I'm pulling all of this out of my… brain without checking and I have not even used gPodder in a while. So I could be utterly and completely wrong here!)