Haven't seen it either.....but check your assembly carefully...it is easy to get lost in it, especially if you have a deep hierarchy, a lot of lists, and a lot of necessary re-groups. ....and when or if that happens, it is likely that you will get errors that are confusing.....
Check carefully the "scope" of all of your re-groups, and how you are mapping the different repeating "lists". ALSO....be sure that the xsd that was crafted for you when you imported the JSON is truly a "valid" schema for the structure you need.
...for example....if you have a JSON document that will have "departments with a list of employees", be SURE that your original imported document truly contained "several departments with several employees"...otherwise the interpretted schema may not reflect what you really want to build.