In a number of combats, it seems like there's an issue with the game engine not displaying squares of a room. Let's say I burst into a room full of Frost Giants. In some cases, only a few squares around the door will be displayed as explored, although all the Giants throughout the room will appear on the map. Subsequent entry into the room will reveal only the tiles within Light radius, although the walls of the room appear to have torches on them when they come into view. In any event, although the Giants are clearly visible and in LoS (with or without light), they should be eligible targets. For that matter, the game won't let you center a Fireball spell (for example) on a tile you haven't "explored," even if it involves throwing into a large chamber with a clear LoS. House rule, or bug?
I'd find it less annoying if the Darkness spell (and the Blindness condition) interfered in any way with spell targeting. But they don't. The result thus feels like an engine problem, not an in-game judgment call on what can and can't be seen. Since other fights in the game will first "reveal" the entire room before triggering the combat, there's an obvious workaround available.