Investigate breaking `@BatchMapping` behaviour for subscriptions

**Describe the bug**
In the Spring for GraphQL repo, a bug was reported for subscriptions `@BatchMapping` https://github.com/spring-projects/spring-graphql/issues/1019

Given the commit/time stamps I suspect the error was introduced in https://github.com/graphql-java/graphql-java/pull/3447

**To Reproduce**
See the Spring for GraphQL thread for a minimal repro. To be investigated
