The Armoury of Zeal perk causes grenade and mine skills to cost 20 Focus instead of charges. The Bulwark of Zeal passive (Defense tree) gives +5% damage reduction for 10 seconds after spending Focus. Unfortunately, this passive does not trigger when spending Focus when throwing grenades. It does trigger appropriately when spending Focus on body armour skills. Most other Focus-related perks, passives, and gear mods do trigger when spending Focus on grenades, so I believe this is an oversight in the interaction.

Thanks!