As of now, Version History deletes old versions after 30 days unless it was marked manually, which is fine and makes sense. However, as you open a more-than-30-days-old file that was not marked and you modify it beyond your History limit, there is no way you can revert back to the original file, at least if you have the autosave enabled, which is to be expected.

I therefore suggest to automatically log a version as you open a file that didn't contain any (more than 30 days old...)