If awarded higher rate of PIP is it correct that it will not be backdated

I sent my PIP review back May 2021 with further evidence as my conditions has got a lot worse. It is now May 2022 and I am still waiting for an assessment. I have contacted DWP numerous times to be told new claims are a priority and I am waiting for my file to be allocated to a case worker but would probably have a telephone assessment when it my time.

I have since heard that if an award is awarded at a higher rate than is already being received then this will not be backdated to the date when you sent the form back as you would if it was a new claim but you would only have any owed money backdated from the time of the decision, is this correct? if so then is this really fair because people could have missed out on £££’s by having to wait so long (no fault of their own) because they would have received the payment if the assessments had been assessed earlier.