Hi Cam,
Thank you for starting up this topic!
I have exactly the same problem on my site and I am still unable to fix it (neither the guys from S2Member support who are trying to help me).
I am thinking of downgrading the s2member plugin to previous version as the last option left. As I remember this bug happened after I upgraded the plugin.
Are you still using the WP registration instead of BP?
Best,
Stefan