I think I found it. I think the trigger that calculates the resources needed (8 of Copper, Iron, Gold, Silver or Lead), only counts Copper. In the save game I attach, I fulfill the requirements (13 forges and 8 metals (2 Copper, 3 Iron, 4 Silver)) but the mission fulfillment does not trigger.
Using the WorldEditor, I crate 7 more coppers to have 9 in total, and the very next turn the mission is completed.
Checking the code I see the checking that the city of the event is still own by the player, and the required number of forges, but I don't see where the required resources are counted