The series_sort calibre field isn't a "real" metadata field, so it doesn't exist in normal book metadata.
Calibre passes CC the series_sort information when you connect as a wireless device, but I don't know if the iDevice CC version uses it.
The best way to solve the problem so that it works in all the connection types is to create a custom column "built from other columns" to contain the value that calibre computes. The template should be
Code:
program: series_sort()
You can then sort by that column, add it to the grouping drawer, etc.