Getting WTC back after PIP appeal success

I have a client who was reassesed for PIP in December and got 0 points.

PIP award terminated, and WTC stopped due to disability element not applying any more

You dont need to be on PIP to get disability element of TC, so i think terminating award immedialtely when PIP stopped was inappropriate

She has just won her PIP appeal, and PIP backdated to December 2016.

Procedural query :

Do you know if Tax credits usually revise the award and backdate to December since PIP was backdated from then, or does she need to make a new claim that will only backdate 1 month?

If only 1 month i will request a late MR/appeal to terminate TC award, unless anyone can advise otherwise.

Any experience with such matters will be appreciated

Thanks