Mirror Image description says
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
The Sacred Flame cantrip does not require the caster to make a spell attack, its effect depends on the target's saving throw instead:
Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 radiant damage.
This implies Sacred Flame automatically targets the creature, revealing the original.
Is it correct, if somebody casts Sacred Flame, the "each time a creature targets you with an attack during the spell's duration" part doesn't work anymore, since everybody already knows where the original is?