OK, this is a little long, please bear with me:
After editing an m4a podcast in any way with mp3tag, even though "ITUNESPODCASTDESC" shows up as still there in mp3tag's editing windows, it disappears in iTunes and on my iPod. Here's the step by step:
Find a podcast that downloads m4a files. Confirm the presence of podcast description text by clicking on the "i" in the row for a single podcast, or choose "Show Description" in the context menu.
Go to mp3tag and use it to edit any of the tags for that m4a file except ITUNESPODCASTDESC. I found the problem when I was editing ITUNESPODCASTURL. Confirm that the text in ITUNESPODCASTDESC is still there.
Go back to iTunes and click on the "i" or choose "Show Description" again. The text that used to be there is now gone. Now when you sync with your iPod the description is gone as well. To make this bug show up, you may have to "refresh" the item by deleting the item in iTunes (but keep the file) and re-add it via "Add File to Library..." or "Add Folder to Library..." through the File menu.
This situation came up because a podcast that I frequently download changes its ITUNESPODCASTURL frequently, so I am trying to edit that field for all the old items for that podcast accordingly so they all show up as one group under iTunes. However I experimented and this bug occurs if you edit any of the tags, not just ITUNESPODCASTURL.
Thanks for the help in advance.