iPhone SE battery dying unexpectedly after upgrading to IOS15

I still use an iPhone SE (2018 era) as a daily phone- and it has been generally fantastic.

However, since upgrading to IOS15 the phone powers down unexpectedly even though battery voltage is reported as high. This consistently occurs after a few hours of use, and when plugged in to restart and recharge, the battery charge level history shows an abrupt drop from >80% to 0% almost instantaneously (see attached image).

I haven't changed phone habits recently or connected to any new devices. The phone is relatively new and the battery health is reported as 97%, with peak performance supported. If I didn't know better, I would suspect that the battery voltage is being incorrectly assessed but I don't know why this should happen after the upgrade to IOS15 specifically. I didn't have this problem before upgrading.

Thanks for any help you can provide.