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It isn't anything to worry about. See the information in About recalibration of battery health reporting in iOS 14.5 - Apple Support including the following:
iOS 14.5 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.
While the battery health reporting system is recalibrating, you will see a message in Settings > Battery > Battery Health. Recalibration of maximum capacity and peak performance capability happens during regular charge cycles, and this process might take a few weeks. The displayed maximum capacity percentage will not change during recalibration. Peak performance capability might be updated, but this might not be noticeable by most users. If a previous degraded battery message was displayed, this message will be removed after updating to iOS 14.5.
When recalibration is complete, maximum capacity percentage and peak performance capability are updated. The recalibration message will also be removed, indicating that the process is complete.