It actually looks like an issue with the paper... to me, it looks like the sizing (assuming this SG paper is sized). I've seen similar things with the watercolour paper I use for alt processes, but never SG paper. Not only can you get spots without sizing where light shows through the paper (especially when it's wet), but you can also get places where the paper is over sized, which shows up as shadows in transmitted light.

Either way, I don't think it's your processing.