I've using a WDC to pull a JSON feed using the following WDC:
I can no longer refresh the extract as the update returns NULL values for a Date dimension called "DateSubmitted" but only for some values of 2016. The JSON feed has not changed, nor did the date field format. All other records fields are successfully pull in Tableau except this "DateSubmitted" field.
You'll find attached the extract before the latest refresh and After refreshing the extract where dates are missing are returning NULLs.
This does not look like a "refresh" to me - you get way more columns after so I assume you've changed your data model.
Can you attach your json files?