Items that give +acuity will give a corresponding debuff to dexterity to compensate on a Nightshade, as their casting stat is dexterity, and I guess somebody thought it unfair for +acuity to affect a combat attribute.
However, if dexterity is at or near the level cap, then the acuity buff is affected by the cap, but the debuff still counts at face value.
For example, a level 11 Nightshade has 16 cap for dexterity items. He's at 115 with gear, and adds a bracer with +1 dex / +4 acuity. His dexterity actually goes DOWN to 112. The +1 dex hits the cap, the +4 is over cap and not applied, but the -4 is applied at full value.
However, if dexterity is at or near the level cap, then the acuity buff is affected by the cap, but the debuff still counts at face value.
For example, a level 11 Nightshade has 16 cap for dexterity items. He's at 115 with gear, and adds a bracer with +1 dex / +4 acuity. His dexterity actually goes DOWN to 112. The +1 dex hits the cap, the +4 is over cap and not applied, but the -4 is applied at full value.