Some context: We're running Dragon Heist and the party is brainstorming on a smart combo game plan to fight the Xanathar himself. In this question I've clarified why in our encounter and generally it may not always be reasonable for a Beholder to point the Antimagic Cone at the target instead of shooting his rays at with a readied action (even if that target is a spellcaster, there is always a tradeoff: "disable magic VS just kill that thing"). So here the party started to brainstorm on some smart ways to buff themselves against the Beholder's rays.
One of the proposed tactics was to cast Mirror Image. I thought this was a cool idea! But then I read the description of the spell again and then stumbled upon this question where the consensus is that Mirror Image only protects against the effects that specifically mention "attack" in their description.
So it seems that, according to the rules, the Mirror Image is completely useless against the Beholder's rays. Because just like the disintegrate spell, the disintegrate ray is not an "attack".
But how in the world should I explain that to my players? 🤷♀️
When reading the Mirror Image spell description, here is what I imagine (it's a screenshot from a video game Baldur's Gate 2, based on D&D): Actually, I even remember playing Icewind Dale / Baldur's Gate video games and remember that when I casted Mirror Image, it always looked like there are identical duplicates, indistinguishable from each other, just like the spell states:
duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real.
Other spell lore like this one also suggests that those images are identical.
So how would my Beholder know which one of the images is the real target? Is it because he's super smart or something? What about spellcasters?
I honestly don't know how to explain it to my party. I'm one step away from saying "this is a HUGE hole in the D&D rules and I'm house-ruling this spell: the Beholder now will have to roll d20 and there's a chance he'll waste one of his rays at one of the illusory doubles". Wouldn't that make Mirror Image overpowered then?
But maybe I'm missing something and there is a nice way to explain the rationale behind Mirror Image not working against the Beholder's rays (as well as against the spells that are targeted at a creature but don't have "attack" in their description)?