PIP Award Review they’ve stopped my award

I moved from DLA to PIP back in 2017. Mandatory review, Appeal, Awarded standard on both.
Award review delayed and an extension date, due to bereavement, for 15th October 2020.
I got all my appeal and Recent/New evidence, completed the form and sent Special Delivery to make certain it got there on time.
Post Office Tracking states delivered 05.28am and signed (named individual) on the deadline date.
I get a letter yesterday to say my award form was late?? and, therefore, I will be paid upto 23rd October. Rang helpline - hours wait - and was told at first my review form wasn’t delivered until 24th....some minutes later changed the date to the 19th October. I explained I had proof it reached them on the date. Was told then that there’s a 4 to 5 day delay from postal room to review team. Explained that that was their problem not mine and no one told me to send several days before deadline because of their backlog!!
Was told they will inform medical review team I’d called and sent form and evidence on time.
My usual payment date was 23rd October but checking my bank statement I’ve been paid £107.10 less than usual. More than a weeks PIP benefit. Doesn’t make sense to me.
What do I do next any advice please.