Seniority Date Fast Formula not updating LOS value

We have a requirement that there is a DFF in Work Relationship attribute which will be updated for Acquisition Date. When this field is not NULL, the Length of Service will be updated based on this date, or else it will update based on Original Hire Date. The Fast Formula is returning multiple lines and not updating LOS value. Please suggest.

/*FORMULA TYPE: Employment Seniority Date Adjustment*/

DEFAULT FOR PER_REL_DATE_START IS '0001/01/01 00:00:00' (date)
default for BASE_SENIORITY_DATE is '1900/01/01 00:00:00' (DATE)
DEFAULT FOR SENIORITY_ADJUST_IN_DAYS IS 0
DEFAULT FOR PER_ASG_PERSON_ID IS 0