I agree.......I would think the chances are fairly small being ph if every single plant except one is normal and they're all fed off the same plate. Even more interestingly.......said plant is directly under a lamp.

I'd move the plant first and see if that helps. Radiant heat causing suffocation/heat burn is not the same as high humidity imo.........although I guess TOO high could suffocate parts of the plant as well.

I wouldn't flush it unless you are sure it's necessary.