Pheww.. Just updated my new Macbook pro 15 inch 2017 to Big Sur, and had the same issue. Battery cycle count is 5 and battery health capacity shows 100%. Warranty till this year end.
Pls tell me its a Beta bug and nothing else 😅🙏🏼
* edited response;
Thanks a lot Irfin ! I just did a PRAM reset and now it shows normal. ☺️