As identified by a colleague, pasting an MB json-ld document into the json-ld playground (https://json-ld.org/playground/) gives a warning about badly formed data:

jsonld.InvalidUrl: Dereferencing a URL did not result in a valid JSON-LD object. Possible causes are an inaccessible URL perhaps due to a same-origin policy (ensure the server uses CORS if you are using client-side JavaScript), too many redirects, a non-JSON response, or more than one HTTP Link Header was provided for a remote context.

he says "If you change the value in @context from https://schema.org to https://schema.org/docs/jsonldcontext.json, then it will magically work as expected"