I have had this same issue as well with a schema that has 369 fields and its causing a lot of trouble. I have 18 sets of records that should either be blank or filled out, depending on whether the record before it was filled out. When I get to record 18, Mockaroo provides an error that record17.lastName doesn't exist.
It's as if I need to create hidden fields for whether each record# is completed or blanked out at the beginning of the file so they impact fields towards the bottom of the file. Are there any hard limits on the number of fields within a schema, or on the organization (ie. nesting) of those data elements?