I have been looking around and haven't found anything that could give me a clue. I used the A5 quest to add a socket to The Gladiator's Bane so I could insert a perfect diamond. Inserted it and for my surprise nothing has been added to the description rather than now socket  is listed as socket .
The merc resistances are neither affected when he wears the armor (my sorc doesn't have that much strength). Is a bug or is something happening backstage that I'm not aware of?