Hello,
I have the same issue, I think it's related to the iOS 11.3 as before all was ok even the external apps on apple store was show 100% but since I set the iOS 11.3
I have this issue
also I reset my phone the battery comes up to 100 % for 3 days
and when my phone restart it becomes again to 99 %
there is something wrong with the IOS
however I check my phone on the MacBook console and the battery health was 100 % with 132 full cycle
I think no need to worry
and wait for IOS 11.4
to find out if all will become good again or not
forget to tell I contact apple store in Brussels and apple support no one know anything about this
all says it's normal and if you want to change the battery we can do it for free