Why does a Fast Formula reset balances to 0 at year end and/or anniversary date?

EBS release 12.2.8

Client is using accrual plans for vacation and sick time. Sick time is accrued each pay period. Vacation time accrues on the employee's monthly anniversary.

Sick time balances are reset to zero in the pay period that includes January 1. The balance on the dates prior to January 1 are zero. When the system date flips to January 1, the balances are restored. For example: during the pay period of 12/28/2020 - 1/10/2021, sick time balances were correct on 12/27/2020. On 12/28/2020, the balance reverted to zero, and remained at zero through 12/31/2020. On 1/1/2021, the balances reverted to the correct amounts. TImecards for employees who used sick time from 12/28 thru 12/21 caused a warning during the Validate for BEE process.