I created a form using Survey123 with a few related tables. When the form is submitted, if there's no data in the related tables, features are systematically created with blank fields. Only some of my related tables are causing this issue, despite the fact that I use the 'null' option in the EsriFieldType column. The other related tables function as expected. Why?
I've seen similar behavior, too. I feel like it's more frequent from the WebApp, but would need to experiment a little to say for certain.
It sounds like your repeats are "optional" in a lot of cases? If so, you can try using the `minimal` appearance. This will leave the fields absent from the form until the user clicks the plus button. My theory is that having the fields hidden will prevent the `nulls` from getting written to the related table's feature layer. YMMV.
Yes this is a known issue.
My project has 3 or 4 tables doing it so hopefully it is coming soon.