The GraphQL mutation is not malformed. I tried reproducing this on master and it worked perfectly fine. There was a bug with the way GraphQL mutations were rewritten to DQL.
This bug was fixed with Fix(GraphQL): Refactor Mutation Rewriter for Add and Update Mutations by vmrajas · Pull Request #7409 · dgraph-io/dgraph · GitHub and is available in all versions after 20.11.2 .
You are able to see this error on Slash because Slash is using an earlier version. Dgraph version of Slash is updated periodically.
@gja can share when the next version update of Dgraph on Slash is going to take place.