This is consistent with other troop spells that have boosts based on effects which are dispelled on spell casting:
Enchant and Submerge are both removed when spells are cast, so for example Undine also won’t get the boost for himself being submerged when he casts his spell because submerged is dispelled on spell cast.
Grand Inquisitor is different because his spell isn’t boosted by temporary effects which dispel on spell cast, his is boosted by barrier which is only removed when receiving damage, and by Whitehelm allies, which doesn’t change ever.