We have several projects with GKE cluster on each. We faced a problem that there is no connectivity when using internal load balancer between pods in project #1 and service in project #2, but service in project #2 is reachable from nodes on all the projects.
Did you setup your Clusters with a Shared VPC? This is because with Shared VPC, you designate one project as the host project, and you can attach other projects, called service projects, to the host project.
You create networks, subnetworks, secondary address ranges, firewall rules, and other network resources in the host project. Then you share selected subnetworks, including secondary ranges, with the service projects. See the following document for more details.