In general indexing for search/filter, does involve a few seconds.
Instead of using the Filter APIs, please try the View a Contact API.
The response body of a successful POST request will have the Contact’s ID. You can use that to fetch the full contact object, instead of relying on the Filter API.
This would be a simple change, if you are hitting the API in the same thread. I am not sure if that’s the scenario in your case.