App proxy route returning 400 GraphQL error

Extensions
1

One of the stores that has installed my app is having issues sending request to my REST API via the app proxy. The requests sent to https://example.myshopify.com/apps/proxy are returning a GraphQL error and not hitting my REST API. What could be the issue?

{
    "errors": [
        {
            "message": "GraphQL operations must contain a non-empty `query` or a `persistedQuery` extension.",
            "extensions": {
                "code": "INTERNAL_SERVER_ERROR"
            }
        }
    ]
}

Relevant part from shopify.app.toml

[app_proxy]
url = "https://my.rest.api.com/api/proxy"
subpath = "proxy"
prefix = "apps"

client code

async function callApi(): Promise<string | undefined> {
  try {
    const res = await fetch("/apps/proxy", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({
        hello: "world"
      }),
    });
    const json: ProxyResponse = await res.json();
    return json.success && json.data ? json.data : undefined;
  } catch (err) {
    console.error("request failed", err);
    return undefined;
  }
}