Jira generates branch names using a naming scheme that includes a JIRA ID. This is also a common practice anyway, like pointed in a comment to BSERV-2925.

Bitbucket has a feature where it "de-punctuates" the branch name when it places it in pull request titles and descriptions etc.

For example, branch name 'PROJ-123_fix_bug' would be turned into 'Proj 123 fix bug'.

This breaks the hyperlinks to ticket that were introduced in BSERV-3090 (as per this comment)