NEST pension integration is two way, meaning NEST also sends data back to us.
If you observe that pension period dates are being changed automatically, it means that you are likely to have the employees in a pension group that doesn't match the settings you're trying to apply in Paycircle.
Ensure that all team members are moved to the pension group that has the correct dates aligning with the setup of the same pension group in the NEST portal. Once actioned, you can then apply the pension dates as required.