Typically a split core is either a freeze situation or a lack of a restrictor in the inlet line to the heater core. I typically bend the large tubing to a 3/8" square opening to mechanically reduce the slug of water hitting the guts of the core under hard acceleration. An Old WW-2 Pontiac mechanic told me about the "Mod" after I failed mine from "hard acceleration testing".
If you had properly recirculated anti-freeze to the core then the core did not freeze.
So did you do your recirculation "fill" with the valve open?
Tom Vaught
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"Engineers do stuff for reasons" Tom Vaught
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