I kinda doubt it's a crack because of the description. Likely a gasket that can't contain the pressure difference when the engine is cold. A cold un-expanded gasket getting hit with 70psi
oil on one side and an un-pressurized coolant system on the other.
Once the engine warms up coolant pressure raises and
oil drops, the leak stops.
If it was a crack when the engine is shut down the
oil goes to 0psi with 15+psi hot coolant. (coolant in oil)