This may happen when you use fork-based workflows, like described below:
new forked repository is created by user A
user A adds new commits to this repo and creates back-merge Pull Request to original repository
if user B tries to merge such Pull Request, these commits are to be added to original repo by user B
commits in such Pull Request are new to original repository and should be validated by YACC
YACC compares commit author name/email with current user B - and they are different, so such commit is to be rejected by YACC
Unfortunately, there’s no robust / guaranteed way to detect and handle such forked commits, so the suggested workaround is to use Require Committer Matches Valid User option instead of Require Matching Committer Email / Require Matching Committer Name ones. It will do a less strict check that author of commit being merged is known Bitbucket user.
YACC of version 1.x had a bug when all commits wrapped by Pull Requests were just not validated / skipped. This behavior is incorrect and was fixed in YACC 2.x, causing described issue in some rare workflows.