Perhaps the gasket just needed more time to soften up after being dry for a long time?
Absolutely, the gasket shrinks over time and hot water makes it expand and seal again. It usually takes 24 hours for the the gasket to repair itself if it is going to (which doesn't always happen).
I never look for new-old-stock gaskets, I always cut new ones from fresh roll-cork or sheet neoprene. New-old-stock cork gaskets dry-out and degrade many times simply by sitting on the shelf. Sometimes even new gaskets wont work if the cut area in the metal has rusted or deformed from moisture, in that case I use Permatex Ultra-blue non-toxic silicone which has never once not stopped a leak.