Is there any limitation with AWS PrivateLink?

I'm implementing a model-based system User -> AWS API Gateway -> AWS PrivateLink -> Network Load Balancer -> EC2 (Apache with PHP). When I load test with the NLB endpoint directly there is absolutely no problem with high load. But when testing with the API Gateway endpoint I get a lot of 500 error codes, Execution failed due to a network error communicating with the endpoint: Connection is closed, execution failed due to a timeout error (although it's a timeout error I get an error after very fast time). I'm wondering if there's something wrong with AWS PrivateLink? Can someone help me, thanks