Object in document being replaced with array of arrays

I am trying to put a few documents into my elasticsearch, but when I do, one of the objects in the document gets replaced with arrays of arrays.
Original document
Document returned from ES
I have checked that the original document satisfies RFC 4627, so it should be supported by ES. I haven't been able to find any other leads so anything helps.

Can you make a gist that reproduces the issue with curl? That'd make it much easier to figure out what is up.