Summary

According to Referencing issues in your development work a merge commit does not associate the issue to the branch. However, merging a branch to master associates data to the issue which is linked to the branch.

Steps to Reproduce

1. In Bitbucket create a branch e.g TestBranch1 and link it to an issue in JIRA e.g TEST-1
2. Create another branch TestBranch2 and link it to another issue TEST-2
3. Merge TestBranch1 to the master
4. Merge TestBranch2 to the master having TEST-1 in the merge commit message

Expected Results

Looking at JIRA issue TEST-1 only the TestBranch1 should be associated

Actual Results

Both branches TestBranch1 and TestBranch2 are associated to TEST-1