"Gain +1d4 elemental damage for 2 turns. Overrides existing bonus. If held, off-hand weapon receives +2 Deflection AC for 5 turns."
I guess it is working as intended, but it is punishing to those using the perk with an exotic double weapon, since they do not receive the +2 deflection bonus. Correct, or bug?
Correct. Two-handed weapons however get the benefit of the +1d4 elemental damage on both sides, something which I believe the dual wielders do not.
It appears that double-weapons don't get the Deflection, but dual-wielder perk lets you replicate elemental damage buffs onto the offhand.
Dual-wielder
Regardless of whether you started as right handed or left handed, you always considered favoring one over the other would hamper you in the long run. Sacrificing the comfort and reliance of a shield you instead enter combat with blades blazing, whether in the form of two singular weapons or a staff for both hands.
Level 1: Gain ambidexterity if dexterity is 13 or above.
Level 5: 5% phyiscal immunity. The effects of magic weapon, flame weapon, darkfire. poison and alchemist's fire duplicated to off-hand weapon when cast.
Level 8: +2 off-hand attack bonus.
The topic is the fighter perk's ability -- missing from the quote -- not how spells interact with the perk.