SSP feature upgrade to Case - unexpected retro entries

We are testing the feature upgrade for Statutory sick Pay absences using the case method.

Employee has 3 Assignments.

Sickness absence 1 - 3/1/2025 to 24/1/2025 already exists.

Run Feature upgrade with transition date 1/2/2025.

Enter Sickness absence 2 - 1/2/2025 to 28/2/2025

Run Feb 25 Retropay.

Retro results show Sick Offset Retroactive and Sick Pay Retroactive entries for the
Primary Assignment and dates within Jan 25 sickness absence period.

Any suggestions for action or an explanation why this happens welcome.

Thanks,
Gavin.