Battery Health

I have an iPhone SE and the Maximum Capacity has been sitting at 86% for a very long time (more than a year). That doesn't make any sense. I can't find anything that addresses this. It gets charged at least once a day (overnight) and is plugged in pretty much whenever I'm near a source. Can anyone advise? Should I replace the battery? Phone still works beautifully and is on ios 14.0.1.