Summary:
With the DVCS integration with Jira Cloud and Bitbucket Cloud. The issues associated with the pull request can de identified via the Jira development panel as well as from the pull request view. But their are no APIs available that lists JIRA issue associated with a PR.

As a workaround available PR API can be used to get PR details, but their is no specific field available in the API to fetch the JIRA issue number correctly