quarterly billing question

We have an award that started in March 2020.  It is cost/cost, quarterly billing in March, June, September and December.  No First billing offset days were entered.  The first expenditure hit in May.  An invoice generated that night (5/5) and then the next invoice came out on 6/30.  The functional users asked why it invoiced 5/5, since it was set up as quarterly.

My hypothesis is that since it started in March and no offset days were used, the system was expecting to generate a March invoice.  So once the first cost hit, it generated that "missed" invoice.  Then it waited to generate the next quarterly invoice in June.