Protected payment on incapacity benefit to esa migration

I was transferred from incapacity benefit to esa 10 years ago and from that date my payment was transitionally protected, meaning I still receive the amount I did rather than the lower rate of esa (support group).
My last two fortnightly payments have been reduced to the standard rate of esa.
I phoned today to ask why and was told it was a mistake and an email would be sent to pay the arrears and rectify. I then had a phone call from my file manager who said there had been a glitch on the system and my file was removed. Upon re-entering it manually, she removed the uplift as it was something I am no longer entitled to ( a child premium I believe).
I ceased to be entitled to this before incapacity benefit ended but for some reason they were unable to cancel it ( I got this in writing and it was a legal overpayment and above board).
I have it on writing that I am entitled to the uplift on every letter I’ve had from esa. I’ve had my original transfer and three reviews and it continues
I believe that the uplift should continue and dwp officers says not.
It’s been referred to a decision maker but I’d like to know if I’m in the right and how to proceed
Surely a system glitch can’t be a reason to reduce my payment
Thank you