PIP reassessment decision letter before AR1 return

I received the PIP AR1 form last month. As the form took 10 days to arrive, I called for an extension which was granted until 2nd of April. I sent a letter a few days later to record the date agreed and the reasons for me asking for the extension.
The initial deadline ran out at the end of last week but I didn't receive the reminder letter despite the agent telling me I would even with the extension.
Today I received a brown envelope. I was panicking opening it thinking my PIP might have been stopped.
In fact it seems to be a decision letter saying 'I have looked at your PIP and decided' then confirming the previous award from 11 March 21 to 10 March 27 (same scores, same award same length as the initial award)
I am now totally confused and wondering what to do for best. I was still working on the AR1 form have not returned it yet.

Is it something that people have come across i.e. a paper based decision made based on old PIP form, ESA forms without waiting for the AR1 reassessment form when one has been sent to the claimant? ( I have had 3 ESA reassessments with medical since my initial PIP award)
I am a bit wary of calling the DWP in case the 'early' decision is a mistake and they revoke this decision.
What should I do about the AR1 form? What are the risk if I stop and don't send it back now? I am not feeling well at the moment and do not have much help with the form due to lockdown so I really could do with archiving all this until next reassessment!
Help please!

I thought I would update on this. I called and managed to speak to someone. The decision has indeed being made without me returning the AR1 form by looking at previous documentation (probably mainly ESA reassessment). The agent double checked with a manager and I do not need to return any form now.
So it can happen!