The issue is with SLA calculation for the correct Actual elapsed time, Business elapsed time, Business time left.
SLA's calculating incorrect business and actual times after a specific date
SLA schedule has the Repeat until field set to a date that has passed. eg. Repeat Until field set to 31-12-2020 and SLA calculations are incorrect/unexpected for all records after this date.
Symptoms: All the SLAs (for a specific SLA definition) that attached after the date set in the Repeat until field, started in the paused state with incorrect timings. eg. business values are all 0.
Create a new SLA with an updated schedule or change the "Repeat until" date from 2020-12-31 to a future date on the existing schedule.
NOTE: Repair SLA is required on the affected task SLAs to fix the timings of any affected records.
NOTE: It is preferable to create a new SLA definition instead of modifying the existing SLA.
Refer to KB0869748 for best practices on changing existing SLAs.