Unfortunately, this issue has persisted for years despite updates to Final Cut Pro X. It's surprising and disappointing that such a basic function isn't working reliably. It seems like you've tried various methods like zipping files and sending XMLs, but the problem persists. Here are some feasible solutions:
Check File Formats: Ensure that the file formats and codecs are fully supported by Final Cut Pro X. Sometimes, even slight differences can cause issues with relinking.
Reimport Media: Try reimporting the media directly into Final Cut Pro X instead of relying on relinking. This can be time-consuming for larger projects but might bypass the relinking issue.
Update Software: Make sure you are using the latest version of Final Cut Pro X. Sometimes, software updates include bug fixes that could address your problem.
Contact Support: Reach out to Apple's support for Final Cut Pro X. They may have insights or be able to escalate your issue to their development team for further investigation.
Ultimately, if you continue to face these issues, it might be worth considering alternative editing software like Adobe Premiere Pro or other editing programs.