This is just a warning and reference for people searching for this issue in the future. One of our servers automatically upgraded to PHP 7.3 due to the version constraint being set incorrectly. Craft compatibility aside, this causes some pretty weird behavior with SEOmatic. The primary symptom I observed was that SEO meta field data would not save (though the overrides were working, no errors thrown).

I suspect this has something to do with string sanitation parameters having changed in 7.3; however, I didn't look into it very much since we weren't supposed to be on 7.3 anyway.

If you're using services like Heroku, make sure you set your PHP version constraint to this to avoid automatic upgrades to 7.3:
"php": "~7.2.0",

This package is deprecated and unmaintained, so I'm just posting this to help save the next person some time.