Hi there,
I recently noticed the same issue with my podcast ("Ponken & Friends") and it looks like you know what you're talking about. Have looked around the web for a fix, and it appears as though a lot of people are having this issue but very few know what to do about it.
I have a Wordpress RSS feed (using the Powerpress Blubbry podcast plugin) that is run through Feedburner to iTunes, and I have no idea where I could even add a PubDate tag (or any tag for that matter).
So far, I've got 27 episodes up there and I know that at least the first ~20 used to appear on iTunes in the correct order. Now, however, if you look up my cast on the App Store, the order is completely jumbled. I apologise for being such an amateur at this. Any help would be greatly appreciated. I'd be interested to know what I can do to fix this going forward and if there is anything I can do to my feed, or even the actual files, to retroactively sort this out.
Here are the relevant links:
http://feeds.feedburner.com/PonkenFriends
https://itunes.apple.com/gb/podcast/ponken-friends/id461730132
The eps are in order in both of the above.
Many thanks in advance,
Leon