DWP /Procedural Rules re: Retroactivity of PIP awards

My question relates to PIP rules for Back Payment after delays in claim reassessment.

Last November 2024, I was reassessed for PIP and was given an award for full PIP Mobility and Medical.

As my Reassessment Period fell in November 2023 and it took 11 months for Reassessment, I was under the impression that Back Payment was procedural.

However, when I contacted DWP and spoke with the case manager regarding backpayment, i recieved a letter today after waiting 12 weeks, that the DWP asessed my condiitions(again) but no backpayment was mentioned.

I was wondering if anyone can shed light on my Question.

Many Thanks