Tempest Shield + Cybil's Paw - Spelldmg increase from block chance not applied

Tempest Shield is definitely affected by the spell damage bonus.
The most likely explanation is that because of the large increased damage bonus from Iron Will applying to Tempest Shield, and it's relatively low base damage compared to something like Molten Shell, the effect of adding another 6% increased damage isn't enough to raise the actual final damage by a full point, and thus no change can be shown.