This is not a bug but by design. It is in the code:
// we go from single event to recurrence: create the recurrence and delete the single event
eme_db_insert_recurrence ( $event, $recurrence );
eme_db_delete_event ( $tmp_event );
Now the reason for this: when converting a single event into a recurrence, I don’t know if the original event could even be part of the recurrence (and exactly to which event the rsvp info should be added) You have to think of a recurrence as a “master” event that just has “single” events under it.
I could add some warning to the edit screen for this case …