I think we’ve narrowed it down to the theme the client is using (a customized version of the Rebound theme).
All plugins deactivated + custom theme: SJB malfunctions
All plugins deactivated + 2015 theme: SJB function
All plugins activated + 2015 theme: SJB functions
All plugins activated + custom theme: SJB malfunctions again
So, it looks like we’re conflicting with something in the theme.
Looking inside both functions.php and theme_functions.php, I see no mention of a “jobpost” shortcode anywhere, so I don’t think there’s an issue of the theme *intentionally* trying to hijack that shortcode, even though it appears to be doing so anyway.
I’m not sure where to take the troubleshooting at this point.