Why failed to set request.body.query in supergraph_service Rhai script

Router
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1

according to rhai API document. (https://www.apollographql.com/docs/graphos/reference/router/rhai#requestbodyquery)
‘request.body.query’ is modifiable in rhai supergraph_service function.
so, I modified ‘request.body.query’ like this.

fn supergraph_service(service) {
service.map_request(|request| {
print(${request.body.query});
request.body.query=“query invalid { _typnam }}”;
print(${request.body.query});
});
}

but, just ‘request.body.query’ is modified locally(in super_graph function only), subgraph query is still not changed.
Why not changed query in subgraph?