Nowhere in the original role suggestion: https://epicmafia.com/role/1459

Or in the role description: https://epicmafia.com/role/15873

Does it say that plumber does not attend mafia meetings. I am pretty sure that plumber was intended to be a meeting mafia as its very weak as a role otherwise and the fix pipes ability is only useful if they have the knowledge of their partners that a meeting mafia would have.