Refracting Bastion ruins poison blasts dmg

I am lvling in Faf swamp and came in here around lvl 240 and began using mass hold and poison blast on the goos and flying skulls. The dmg of PB was around 500-1300 a hit on all of those mobs. when i hit lvl 300 i equipped my mage sigs, seers saber, and my refracting bastion. first fight after equipping those items i notice my dmg with poison barrage went down to 30-150 a hit on each mob. i tried un equipping all the gear i equipped at lvl 300 and the damage went back up to 500-1300 a hit. i requipped the rings, dmg stayed 500-1300. requipped the seers saber dmg was the same at 500-1300. then i equppied the RB and the dmg went back down to 50-130. i was able to reproduce this in combat and out of combat fight after fight. would there be a reason why the refracting bastion would be lowering that spells dmg?

Good discovery… PB does appear to do less dmg with RB equipped… that is the shield that my wizards use! I tested it out on those tester NPCs north of Kurz.

Edit: Testing out running ToD WITHOUT shields equipped on 3 wizards… they do much better… they don't get wiped out by the PB from the diviners/witches/magicians.