I'm unsure what happened in the definitive edition, but the Red Prince with maxed Persuasion and the Pet Pal talent never got the option to tell the Royal Fire Slug that Braccus Rex is long dead. We were given a sticky note but no dialog option. This is a game started in multiplayer amongst 3 players. We are nearly done with Joy Island. (This worked in the classic edition in 2017.)
Mods or no mods, nothing seems to have changed.
This is a problem many others have seemingly had, but we aren't sure what triggers learning/knowing that Rex is dead.