I have no idea which monster you're talking about. The only thing I can think of is maybe you're mistaking a spectator for a beholder - they are definitely not the same thing.
However, they have, and I quote:
Spell Reflection. If the spectator makes a successful saving throw against a spell, or a spell attack misses it, the spectator can choose another creature (including the spellcaster) it can see within 30 feet of it. The spell targets the chosen creature instead of the spectator. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.
Have you seen the spectator display it?
