Validating implicit grant JWTs in React before API calls

We are using implicit grant for a React agent desktop app. The token arrives in the hash fragment and I parse it with jwt-decode. When I send the token to our backend proxy, it returns 401 Unauthorized. Is there a specific header I need to add or is the token format wrong?

const token = getHashToken();
const headers = { Authorization: `Bearer ${token}` };
fetch('/api/v2/users/me', { headers });