I’ve never cut a core apart but I presume the fluid is flowing into the top of the core’s tank and right back out the outlet tube instead of flowing through the core tubes or only flowing through very few of the tubes… Is that possible???
Not on any of the heater cores I have seen. The only way to get from the inlet “tank” to the outlet “tank” is through the core capillaries. It’s a smaller version of the primary radiator for your engine. If you have good flow from inlet to outlet, it has to be passing through the core.