I was using 6.5.4 with nested item-sets and it was working.
Now it is not working on 6.6.0-SNAPSHOT with nested item-sets, The server log says the is not valid and the content studio can’t load this content-type. It seems to work when I remove but I need to use occurrences.
The only reason I want nested item-sets is to structure the JSON output. The data is all flat when using field-set.