It looks like the XLSForm provided is using a submission URL and the survey may have been built based on an existing feature service. Just to confirm are the relationships between the parent layer and related tables 1:M based on globalID to GUID? Please see this documentation for more information.
This did the trick! Like your example, I had to populate the relevant fields for ALL the fields within the repeats, and not just the repeats like I had.
Thanks for posting this solution!