The summoner unique shoulders 'Feast of Cankers' are bugged and do not increase the damage done by summons.

I tested multiple times with my level 90 Livor Mortis summon.

Damage without Shoulders equipped:
78k-80k per hit

Damage with unique Shoulders equipped:
78-80k per hit.

The results are also the same with two 'Feast of Cankers' equipped.