This is sort of a bug as well.
I went to update a students personal email which is the only email in the system at this point. I edited it to move from a plnu to gmail. When I hit save, the plnu email remained primary even thought it was no longer in the system.
It seems to me that if there is only one email in the system, the primary email should automatically follow that email. It seems error prone for someone to change the email field on edit and the primary email to remain an email that is not loner in the system.
That seems like it could be complicated, but it seems needed. If too complicated, maybe there is a way for a box to pop up that says "Make Primary? Y/N" whenever an email address is updated, especially if it is the only one in the system.