Jira issue keys can only contain uppercase letters.
The check in Bitbucket for the issue key allows lowercase issue keys but the 'linkification' only works with (valid) uppercase issue keys.

The Jira specification will not be changed, which is why BCLOUD-9615 was closed as Won't Fix.
However, the check in Bitbucket should be fixed to reject incorrectly formed issue keys when the require issue keys option is enabled.