Could be, but it could also be a fluke. Perhaps everything after the first file-index file is corrupted, and you only restored things from the first one, or perhaps maybe the entire diff-index can be so large that it's not actually recoverable once you grow it to the size you want.

There are a myriad of easy-to-miss failure scenarios that don't appear in small-scale tests. Only way to ensure you can restore your backups, is to restore your backups