I glanced at the Navy Foundry Manual.
None of the foundry books completely agree with each other on all details, but the trick is to find a method that works with the setup and metals that you are using.
It is not really a right or wrong thing since conditions can vary so much with furnace types, metal types, sand types, etc.
The only photos they had with metal with holes in them were labeled "gassy metal", don't ask me what gassy metal is, and sand too damp.
Both photos show holes, the sand holes being smaller and closer to the surface, and the gassy metal holes being larger and uniform throughout the piece.
Another photo shows a similar defect from a "damp ladel".
You could always pour the metal into a steel ingot mould and see what you get.
That would rule out or verify a sand moisture problem.