Ok, so I've narrowed it down to when we use the reset password page.
This is the use case where we get the issue:
1) First we use the registration form to send out the invitation with the login and a temporary password, and link to the password reset page.
2) The member clicks on the link and login using the login and temp password, then set his/hers new password.
3) The email address is still there at this point...
4) Then the user right after change his/hers password again, the email gets blanked out…. it’s not 100% consistent but pretty close. Just fyi, I don't think it has to be done (the reset) right after you login the first time, but maybe in the same session.
Let me know if it helps.