It would appear that the operation name for subsription requests is ignored.
Creating two operations using the GraphiQL editor can create something like,

query MyQuery {
  __typename
}

subscription MySubscription {
  count
}

but when executing MyQuery, the server responds with

[
  {
    "message": "Must provide operation name if query contains multiple operations.",
    "locations": null,
    "path": null
  }
]

Indeed, just executing the simple subscription operation

subscription MySubscription {
  count
}

results in the server responding in the console with no operation name:

[2022-01-26 18:14:12]: No operation name
subscription MySubscription {
  count
}

Whereas doing the same with a query operation

query MyQuery {
  hello
}

returns the debug output

[2022-01-26 18:14:55]: MyQuery
query MyQuery {
  hello
}