Welcome to the forum. Not familiar with that specific valve but it sounds like a multiport from your description. If it is then a couple things I would suspect are internal cracks and/or possibly low spring pressure. I seriously doubt the spring but something to consider. If it is weak the water pressure could cause the part operated by the handle to lift off the gasket. If you didn't already, I'd take it apart again and be sure the gasket didn't lift off someplace and get a light and look in all the passages for cracks. Not much else that can go wrong.Keep us informed. Anyone else with ideas?
Al