Family Sharing Communication limits don't block spam

Why are spam text messages being delivered to an Apple Watch when "...allow communication with" "Contacts Only" enabled? I just discovered that messages are being delivered to my 12yr olds series 8. This device has cellular data and is paired to my phone using Family Sharing.

I understand that there are additional filtering options for Messages on the iPhone, but they seem to be missing from the Apple Watch.

"Only contacts" should mean exactly that.