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