FWIW you do not need to use GroundProjectedEnv directly from three/examples/jsm/objects/GroundProjectedEnv when using with react-three-fiber. Instead use <Environment ground /> from react-three-drei. I do not see any seams when using the Environment component: bold-wind-b5krr1 - CodeSandbox
If you really want to use the GroundProjectedEnv class, then use this a base for your implimentation