Branch merging function in bitBucket does not work

The function to merge branches in bitBucket does not work anymore. I will merge branch B to branch A and then I will receive a confirmation of the branches being merged, but when I do a diff between branch A and branch B, then they are different.

Step-by-step:

1. Go to bitBucket repo > Branches > Select branch B.
2. Change destination branch from master to branch A.
3. Press Merge. Notice the confirmation message "Merged B into A".
4. Check for diff between branch B and A. They should be no diff, but in fact they are not the same.