The wall in the top circle on your screenshot is dark because event #17 picked a dark FOW sprite nearest to it (the one that's on the right with opacity 80). So the wall sprite opacity is only 20.
And that FOW sprite is dark because there is no line of sight to it. LoS behavior checks visibility in a line from the origin image point of one sprite (player) to the origin image point of the second sprite (FOW). In this case there is just a tiny portion of the corner of the wall that's blocking the sight.
The red oval on the right on your screenshot - the wall is bright because it's part of the same Inner_DS sprite. You can't change opacity for a half of the sprite :)
Anyway, I think the method you are using is really ineffective and too complex. I would replace all those hundreds of sprites with a single tilemap and use a separate round mask sprite for FOW, pinned to the player. You can use a trick with a shadow and blend modes from this post: