I tested Negation Rank 8 inside the Stronghold on preview using ACT and the answer is yes, it does provide additional damage resistance after the defense cap.
You can see the effect in this screenshot, the first attack have the default effectiveness for capped defense (it's 25% due to Deflection, but would be 50% otherwise). Effectiveness of the following attacks decrease as more Negation stacks are applied:
Comments
You can see the effect in this screenshot, the first attack have the default effectiveness for capped defense (it's 25% due to Deflection, but would be 50% otherwise). Effectiveness of the following attacks decrease as more Negation stacks are applied: