I agree with Puteri. As also stated here in this documentation that’s expected.

AlloyDB instances accept connections on two TCP ports:

• Port 5432, the default PostgreSQL port that applications use to connect directly to the instance.

• Port 5433, which connectors, including AlloyDB Auth Proxy use to connect to the instance.

If you have an outbound firewall policy, make sure it allows connections to port 5433 on the target AlloyDB instance.