How are episodes sorted?
I have "Killing Even" on Series Pass. Its last episode of season 3 aired on May 31st.
All the previous episodes prior to that are sorted in the reverse order in my app.
A more recent "teaser" that aired on September 29th, however, ended up at the bottom of the stack.
Is it because the teaser doesn't have a "season number" that the app needed to refer to?
Yep, they are considered extras and placed into a separate section. If they have a season number but no episode number, they are kept in the appropriate season section.