Preventing PostSave Looping
As of Formbird v3.1.6 (verified), there is nothing special to do to prevent PostSave looping.
Any update to the document just saved will no longer trigger a postsave loop which we need to prevent. Just use the function (ft3.)updateDocument as usual.
If there is no overriding need for the self document update to occur in PostSave, move the process to a PreSaveServer ruleset. The document can then be updated by just changing ntf.document, and there is no need to actually call updateDocument at all then.