JSON requests send by GraphQL sampler contain wrong Content-Type header:

    Content-Type: text/plain

which instead should be

    Content-Type: application/json

by default.

One can of course use HTTP Header Manager to set the correct value for all requests, but I think this is only a workaround, as:

  * the default should not be invalid (text/plain Content-Type is invalid for conforming GraphQL server),
  * when mixing GraphQL queries with "standard" HTTP Requests with other content types, adding HTTP Header Manager might require more test reorganization

So I'd suggest just setting application/json as the default content type for GraphQL Request Sampler