BuddyPress.org

While I was going about turning on error reporting I decided to try to remove my custom member theme from the /member-themes/ so that the only theme in the directory was the default /buddypress-member/ theme and the problem was fixed.

Through a little bit of testing it seems that none of the member themes, active or inactive, should have the same name as the WPMU themes. After initially deleting the custom theme I re-uploaded it and the problem came back as expected, however renaming it to something unique made the problems disappear.