You right, 2x40w shouldn't cause bleaching. If anything, it should simpley get a darker brown. If the shroom has detached it does not mean it's dead, it's simpley looking for a better place. They can easily move about when they want.
I would still suggest trying to move the rock to a differnet area of the tank that has less light and low water flow and see if they do better. The one that has unattached itself can also be placed elsewhere with the same idea in mind.
How are the water parameters?