Synchronize labels from issues: When changing the referenced issue, the labels from the old issue are still synchronized

Summary

We have opened a PR for an issue that was in the wrong repository. After migration of the issue to the correct repository, still the labels from the old issue were synchronized (which was meanwhile marked as status/invalid due to being in the wrong repository).

Steps to reproduce

You need 2 repositories on GitHub.

1. Enable "Synchronize issue labels" in gitmate (for both repos)
2. Create an issue in repo 1, and attach a label to it.
3. Open a PR referencing this issue in repo 2.
4. Open a new issue in repo 2, and attach a label (different from the issue in repo 1).
5. Inside the PR in repo 2, change all issue references (commit messages + issue descriptions) to the issue in repo 2.

What is the current bug behavior?

Labels from issue in repo 1 are still synchronized.

What is the expected correct behavior?

Because issue references changed in the relevant PR, synchronization of the labels should stop from the issue in repo 1.

Relevant links and/or screenshots

Happened here: https://github.com/coala/coala/pull/5100

Output of checks

This bug happens on GitMate.io