Is there ambient occlusion in the scene? It could be activated in a post process volume unknowingly, and if so, could be how the blotches are showing up. Are there overlapping alphas / planes in the splintered / broken areas? It would seem inconsistent with the blotches because the geometry is so close together, but it might actually be AO via other geometry within the AO setting’s range of those planks. It’s a longshot, though, I admit.
How did the 2 planks (narrow one, and the shorter, 2nd one in the opacity map photo above) get fixed almost completely? There’s not much blotching on those.