Reprisal (-10% damage on enemy) on raid wide abilities already casted but not blown

Like in current moogle roaster - Containment Bay P1T6. That raidwide "distance" aoe ("proximity" actually) which is placed on ground almost at the start. Does its damage get reduced if I use reprisal on boss AFTER the aoe was placed? Pretty sure there's other similar conundrums in the game beside the aoe type. Lazy to test myself, thanks.

General rule of thumb is you need to apply, Addle, Dismantle, Feint and Reprisal during the cast or before the action if no cast bar, for the mitigation to apply.