Is it a super old bucket? There used to be the "US Standard" which was kinda its own S3 region thing before the aws regional naming system was established. looking here: https://hidekazu-konishi.com/entry/aws_history_and_timeline_amazon_s3.html. Looks like it was renamed to us-east-1 in 2015.

maybe if the bucket is older than that the API might still refer to US?