it's uncommon. i order special meals on all international flights (usually hindu vegetarian or asian vegetarian) and it's there 95% of the time on United. once in a while it somehow gets kicked out of the booking, sometimes when i am rerouted but sometimes just randomly -- which seems like what happened to you.

when the meal appears on the manifest, it's always been loaded.

special meals are often disguisting, but that's a different issue .