Access to fetch at * from origin * has been blocked by CORS policy, even though CORS policy explicitly allows it - express

Question

I am performing a graphql query from my client to my server and am getting the following error in the browser:
Access to fetch at 'https://my-server.com/graphql' from origin 'https://my-client.com' has been blocked by CORS policy:
Response to preflight request doesn't pass access control check:
No 'Access-Control-Allow-Origin' header is present on the requested resource.
If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
I once got this same error in the past which is why I added the following to the express middleware on the server which solved it:
const app = express();
app.use(cors({
origin: ['http://localhost:3000', 'https://studio.apollographql.com', "https://my-client.com"],
credentials: true
}))
However without any changes that I can reasonably trace back to this issue the error is occurring again. The strange thing if I run the client and server on my local machine (on localhost:3000 and localhost:4000 respectively) the error does not happen.
Edit:
On the client I use a library called GraphQL Code Generator to generate hooks which execute the relevant queries. One of them is for instance this one:
export function useUsersQuery(baseOptions?: Apollo.QueryHookOptions<UsersQuery, UsersQueryVariables>) {
const options = {...defaultOptions, ...baseOptions}
return Apollo.useQuery<UsersQuery, UsersQueryVariables>(UsersDocument, options);
}
And to execute the query: const { data, loading, error } = useUsersQuery()
I am using the following package versions
apollo-server-express#3.11.1
express#4.18.2
cors#2.8.5