Why is my branch mapping not applied for a new branch?

One reason could be that you have translated your issue types to a non-English language.

Jira sends the English issue type when creating a new issue from Jira to Bitbucket, e.g. issueType=Bug. If there is a translation available for that issue type, and the current user has chosen a non-English language in the user profile, Jira sends back the translated issue type for REST requests, e.g. “Fehlerkorrektur”. This causes that the app to not be able to find the branch mapping with the translated issue type.

The only workaround is to use non-translated issue types at this point in time.