UK Nation Insurance is not splitting for retro hires

Summary:

Upon hiring someone after the payroll run for a period, the system is triggering retro hire event action. Also, the elements are getting picked up in retro process, But in next periods payroll, NI and PAYE are not splitting but are being calculated on the gross pay as a whole for previous + current period.

So let's say,

employee was Hired on 1st May 2025 after payroll run for May. with annual salary of £48000

We ran retro process in June and the employee was picked up in that process having salary element with value as £4000 for May's run.

In June's payroll, the system is calculating NI and PAYE on £8000. Where as it should be calculating £4000 + £4000.