If you enable s3 enpoint in your route table, it's kind of tricky to know if the endpoint is really working. Two things to validate:
1) traceroute tcp before and after (traceroute -T s3-us-west-1.amazonaws.com 443)
You will see more hope when endpoint not activated
2) try an s3 sync cross region with enpoint activated : it should failed since it's not supported (yet @ 2017-05-02)