AWS API Gateway as an http proxy to ALB
Try directly accessing https://my-api-lb-111222333.us-east-1.elb.amazonaws.com from your web browser and you'll get a similar error.  The SSL certificate is not just used for encryption -- it also verifies that the host is not an impostor, by being consulted in a check of whether the hostname used to access the target matches the hostname of the certificate that's offered by the target

Because amazonaws.com is not your domain, you can't get a certificate for that.

You need to assign a domain name from your domain to the ALB in DNS, e.g. origin.example.com, and point API Gateway to that name, not to the system-assigned hostname of the ALB.