When making a backup copy of your Saves File...

@Wholesome_Goth  If the backup copy already has the issue, then the issue will persist.  If the backup copy didn't have the issue the last time you opened it, then it depends on whether the underlying problem is present.  For example, the backup could be pre-patch and have some data that was fine at the time, or at least not obviously problematic, but that the newest patch changed, breaking the save.  The only way to know is to test.

As a random example, imagine you deleted Father Winter in your backup save, but at that original patch level, the game had no trouble spawning a new one when required.  But at the current patch level, the game can't properly spawn a new one for whatever reason.  The backup save was originally fine because the next time the holiday came around, a new FW would spawn.  But now that that process fails, the backup is broken.

This would obviously be an in-game bug and not just the result of an update, but I can't think of a more easily-described example at the moment.