I am finally able to reproduce this issue on 20.11 branch. This bug is already fixed but not available in 20.11 and will be available in 21.03 release coming this month.
I still need to investigate why it works with some part of your schema that we are using earlier.
We had similar kind of bug previously also Custom field resolvers are not always called, Custom Query not working with Float and Integer types
We made many changes related to JSON encoding in master branch which fix such type of bugs.
These changes will be part of 21.03 release and currently available only in master branch only.
PR:perf(GraphQL): Generate GraphQL query response by optimized JSON encoding (GraphQL-730) by abhimanyusinghgaur · Pull Request #7371 · dgraph-io/dgraph · GitHub