Installed a new battery in an iPhone SE 8 (iOS 13.3) days ago. Did the job myself and then calibrated it carefully (as spelled out here) and all was well—Battery Health Maximum Capacity was 100%, for a couple of days.
Since then, Maximum Capacity has fallen precipitously—to 91% now, 8 days later.
My usage is not unusual—I routinely turn off Bluetooth, optimize battery charging and run the phone in low power mode. And use it mainly to read mail and check Facebook (almost all reading, no video).
What would kill a battery so quickly? Wild theories welcome!