V3 Seniority Date HDL DateStart field

We are currently implementing using V3 Seniority Dates,

In Loading Seniority Dates, we are using SeniorityDates under Worker HDL Object.

Just a few query :

Date Start Field → We have currently loaded this as a date different from Hire Date, but upon checking in the PER_PERIODS_OF_SERVICE table, it is defaulting as Hire Date for the employee. Also , in PER_SENIORITY_DATES_F the field is not exisiting

• Is this an expected behavior?
• What is the practice if we want to load a different date vs. the hire date (ie. Union Seniority date)

Note : We wanted to reflect value of this DateStart / Union Seniority Date in the Seniority Date page.