PIP Review Start Date of New Award

I wonder if anyone can help me to understand the DWP's policy on back payments after a PIP review.
My husbands last PIP award on was due to end 29.04.24 having been extended during the covid period. He had been receiving standard rate for both components because of mental health difficulties We were sent the review paperwork in December 2022 and returned the how your disability effects you form on 04.01.23. During that month my husband had an Autism assessment and in February received a formal diagnosis of Autism with recommendations of how to support him. As soon as we had this I contacted the PIP enquiry line to let them know of this change in circumstance and forwarded the report to them in support of the review. Eventually on 17.01.24 a decision was made and the award was increased to enhanced rate of both components which obviously we are really pleased about.
My issue is that I would have expected that the award would have been backdated to the time of review which is when all the evidence was submitted and then to have received a back payment for the period up to the date of the decision. I rang the helpline to question this and after checking with a case worker the agent told me that there had been a "difference of opinion" and when this happens the award starts from the date the decision is made.
I have asked for a M.R as it seems to me completely wrong to make any change in award at the date of decision and not from when they gained the evidence to make that decision. If they had they not taken over a year to complete the review then presumably he would have been receiving the enhanced rate for a longer period.
I am wondering if anyone has had a similar experience or can clarify the rules regarding this. I plan to go to appeal if necessary but would welcome any advice. Many thanks

Thank you so much for your reply in which you were much more articulate than the DWP agent I spoke to on the phone. What you have told me makes a lot of sense. I am still confused though as after our previous review which took about 10 months to complete, my husband went from no mobility to standard rate and received a large back payment which I assumed was from the review date.
Regarding the M.R I was given conflicting advice on the phone but was told by the agent to ask for one if I did not agree.

Hi, just to update . After reading your advice I phoned Pip up again to query my already submitted M.R form. I was told that a M. R was definitely the right way to raise my concern, and so far no-one from DWP has told me that an award from date of decision after a review is standard practice. I was then put through to a manager who explained that start date of the award is up to the decision maker to set although they have to justify this. After reading me the D.M's reason I am still not satisfied that it is either fair or logical in any way for my husband's award to start from this date so I am now waiting for the result of the M.R.

My review was due Dec 2020 but extended to 2021 then 2022, then 2023. I received the forms Dec 2022 and returned Jan 2023.
I finally had my review with an improved allowance (from standard living to higher).
The auto phone system told me the amount and next payment date, the amount mentioned didn't include any back payment so I held on to speak to a person.
The gentleman confirmrd, i am entitled for the difference to be back dated to Jan 2023 but he's not sure why it's not on the system.
He done some "admin stuff" yesterday and when back in work on Monday will call me and hit "the button" for bacs.
If i hadn't called i and questioned it, i wouldn't be getting thousands of pounds next week - the whole thing seems confusing