More of a workaround than a proper solution to my problem, but when I use the parameters in the URL directly instead of in the query argument it works as expected:

  const fromISO = from.toISOString()
  const toISO = to.toISOString()    

  const { data, error } = await useAPI<RateResponse>(
    `/users/${userId}/projects/${projectId}/rates?from=${fromISO}&to=${toISO}`)