Auto-enrolment while 'Acting Up'

I have recently started a year's maternity cover as a secondment, 'acting up' in a qualified role which has a considerably bigger salary.


Having received my payslip today, I notice that it has been allocated as my receiving my main role salary, with a separate 'acting up' payment added on top. This means that my auto-enrolment pension (my contributions and my employer's) is still calculated on my original/main salary, despite my receiving a higher wage for overall.


Does anyone know if this is the 'normal' way of operating something like this? What would the differences be in terms of pension contributions if my employers had appointed an external candidate on a year's contract?


If it's normal practice then I'm not going to complain too much when I get to enjoy a higher salary for a year but it feels a shame to miss out on the higher pension contributions (especially given that the balance between the pensions contributions and tax means that I end up with the same take-home pay either way.)