Airpods Max battery life suddenly much worse

My Airpods Max are 13 months old and so just one month out of Apple Warranty. Until a couple of weeks ago they would happily last a week on standby and maybe 15 hours in use.

Suddenly that changed - they last less than ten hours on standby, and less than five hours in use.

I sent them for a replacement battery under a third party warranty (Greenbridge) that I purchased at the time I bought them - but the new battery has made any difference, battery life is still very poor.

I have factory reset. I have disabled head detection. I have tried to switch off "Find my..." but it seems to re-enable itself without me doing anything.

I have found that if I unpair from the iphone, then charge, then the Airpods maintain the battery charge for much longer - so it seems to be extra "chat" between the iphone and the Airpods that is draining the battery, rather than the battery itself being faulty.

Given the options already investigated, it seems likely that this change is due to a software / firmware issue, but I have tried speaking to Apple Support twice - they are basically not interested because they are one month out of warranty. They suggested I send it back for another battery replacement, which seems very unlikely to fix it.

Anyone else experiencing this, and if so have you worked out how to fix it?

It is so disappointing that such expensive headphones are basically end of life after just one year.

I'm using them with an iphone 14 pro max on iOS 17.0.1 and ipad pro on ipadOS 17.0.1 and the Airpods Max are on Firmware 6A300.