The "html" links for version 2 resources seem to have the hostname
api.bitbucket.org meaning that the link points at a resource that returns
404 Not found.
For example https://api.bitbucket.org/2.0/repositories/pypy/pypy/pullrequests shows a pull request whose "html" link points to https://api.bitbucket.org/pypy/pypy/pull-request/191. Correcting this to https://bitbucket.org/pypy/pypy/pull-request/191 produces the expected resource.