Backdated Severe Disability Premium

Hi I wonder if anyone could advise re the above? My brother had a carer until January 2021 when his carer had to leave. He informed the relevant dept and the Carer's allowance was stopped then. Recently I have been helping my brother with his benefits recently and noted that he was advised in a previous PIP award that he may be eligible for Severe disability premium. As he had a carer at the time he couldn't access this in 2019 when he was awarded PIP. However he has now been without a carer since January 2021. After many months of trying to find the right dept to deal with it he has been awarded Severe disability premium since June 2023.

I have had to contact them 3 times to enquire about back payment from 2021.

On the 1st occasion I was informed he wouldn't get the back pay because it was too large a sum!

On the 2nd occasion I was told that information was incorrect and if he was entitled to it he would receive it. 

On the 3rd occasion I was informed that as they have no record of him informing them that he no longer had a carer, he would not receive any backpay!  I asked why the Carer's allowance was stopped if they hadn't received information the carer had left in Jan 22021.They could not answer that.

My understanding was that a person should receive it automatically if they are entitled? Gov.uk states:

You do not have to claim disability premium. If you’re eligible, it’s automatically added to your relevant beneft.

So I am a little concerned and confused as to how it can be refused.  Will I be able to appeal on his behalf?

Apologies for the long post and many thanks for any advice.