Slapping weight on the front of a racing bike results in exactly the handling issues you experienced. Glad you had a fork on hand that would work.

This is why bikes intended to carry loads in the front need to be designed to carry loads in the front.

Front end geometry is one of the fundemental differences between bikes designed to be pack mules (audax/brevet/rando, touring, porteur, etc.) and racing bikes. Remember, cross bikes are racing bikes. They're designed for off-road racing. As the intended load in front increases (as is does from audax, to touring, to porteur) the geometry changes need to be more pronounced to compensate.

This is also what makes these bikes handle funny without a load on the front. My audax bike has to be held down in corners--the effect is subtle, but it's there. Otherwise it pops up like cork and straightens out. That's the effect the geometry has to keep the bike from falling deeper than intended into corners when the front is loaded.