See Invisibility isn't bugged. It's just bad.
Rather than being actual See Invisibility. It's more like "Investigate" whereby at the start of a character's turn, enemies nearby simply have to make a DC 13 DEX save to remain invisible (And given that not only is this encounter in Act 3 so is high level, it's also with characters that are DEX based so their DEX saves are very high)
As a result, the most effective means of countering invisibility, is to spam AoE effects - Especially ones that still work through saves. Such as Wall of Fire and Insect Plague.
With the most effective method being to have a Cleric wearing the Luminous Armour using Spirit Guardians (Radiant) - As not only does this ability provide a constant AoE that still deals half damage on a save, but the Luminous Armour means that any time you deal Radiant damage, EVERYTHING in the area (Besides allies) gets inflicted with Radiant Orb which prevents invisibility.
