The official explanation is under silvered it says "some", then in the monster manual some of the monsters say under resistances "from bludgeoning, slashing, and piercing non magical weapons that are not silvered", while other monsters most of them with resistance vs weapons say resistance "from bludgeoning, slashing, and piercing non magical weapons."

So it seems pretty clear to me that the monster will call out if a non magical silvered weapon will bypass it's resistance.

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