Im not sure why .. but... if you click on the last toggle on the monster, the one for Effect (not sure what the right name is , but the ones that do target; attributes; Offence) edit the effect line in the CT to add an exclamation mark just before the word silver - i.e. !silver - then the effect works, immune to anything that isn't silver.
But when I edit the NPC, and add the exclamation mark in, it doesn't travel with the effect on the NPC sheet , but I have no idea why
So a manual work about, rather than a fix