I don't know whether this is relevant to iPads but in Preferences - Tweaks there are 3 items to help control sorting.
- Control sorting of titles and series in the library display
- Control formatting of title and series when used in templates
- Set the list of words considered to be "articles" for sort strings
Have you checked what they are currently set to?
Edit: Sorry, having reread your post I suspect you already knew this.
With the Series field, I just set it to the exact value I want to see on all my readers.