Constitution bonus from Durance's robe doesn't affect Durance. When robe equipped, his CON is still 13 and detailed CON list shows some weirdness(screenshot attached). But when robe unequipped, his CON drops to 11. When robe equipped on other party members, bonus applies correctly.

 

Screenshots and savefile.

 

Edit: After restarting game Durance got +2 CON bonus.

Edited February 29, 20169 yr by Nicholas

Remember that stats boosting items do not stack. You probably have some other equipment that gives a better Constitution attribute.