You can have multiple reservations for one ticket or also multiple tickets for one reservation.

Looks like UA started a new reservation number for the missing segment. You should be able to ask united for the reservation number and check this reservation.

Also the follow on reservation should not cause any problems, since you are going to fly all segments in this reservations.

Usually you end up having problems when there are segments in one reservations which you dont fly, since this triggers often the cancellation of the follow on segments.