Can you recalculate your tax & compare to what was deducted? I had an issue this year when I reassigned tax credits between myself & husband and somehow we ended up underpaying. I didn't notice on our payslips, if I recalculate we did indeed underpay. They are very slow to come back !
It might be a prior year adj. They went back into mine for an underpayment in 2016 that was supposed to be on 2021 but got removed from my 2021 cert. They had made a mistake too on the calculation so I'd definitely follow it up.