Well there is a difference… Kiril and Boldtusk just pump the attack value. The skill of Brienne is different, everytime you are hit the attack value further increases.
I’m not “official” but it we know that this is working as intended. Very early the buffs stacked (hence the different icons), which proved to be over-powered. So there was a change (summer of 2017) for Brienne’s bonus to overwrite, and be overwritten by, the standard attack bonus (e.g. Kiril).
There are currently two attack buffs that do stack on the Brienne/Kiril generic +attack:
Wu Kong’s Gambler’s Stance
Tarlak’s Martial Prowess
Another related buff set is +critical, which has a chance of doubling normal damage. This buff is from Melia, Gregorion and Ares.
Doesnt give details on brienne attack buff stacking correctly tho. Assuming it was stacking with all and was updated to overwrite/be.overwritten with Kirk and boldtusks attack buff