Apple Support note:

iOS 14.5, coming later this spring, includes an update where the battery health reporting system will recalibrate maximum battery capacity and peak performance capability on iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max to address inaccurate estimates of battery health reporting for some users. Symptoms of this bug include unexpected battery drain behavior or, in a small number of instances, reduced peak performance capability. This inaccurate battery health reporting does not reflect an issue with actual battery health.

Have an iPhone 11 series phone? Read the linked note.