So technically, this looks like a split fare, GSO-STL back to back with STL-GEG, which was cheaper than any GSO-GEG published routing.

That said...was there anything on dl.com saying "warning, this is a split fare, many things like SDC & miles for an upgrade will be more complicated?" While we know that here, one can't expect people to recognize that based on a couple of letters in parentheses being different.