The documentation for setting up pipelines, specifically when adding a known host address here, mentions Valid IP addresses for Bitbucket Pipelines build environments but none of those listed worked only ones under Valid IP addresses for bitbucket.org, api.bitbucket.org, and altssh.bitbucket.org worked.
Error when trying to add an IP from the Bitbucket Pipelines build environments section:
Unable to fetch fingerprints, check host SSH connection and try again
