I've been using the Principled BSDF node for the last few months. During this time I have noticed a fresnel transition between transparency and reflection which is too sudden. Below I will document examples which work and examples which exhibit the broken behavior.
Hi Brecht, thanks so much for the suggestion, the problem seems to be that my normals in particular cases were flipped. I can reproduce it in blender by manually flipping the normals. Please close this for now and I will re-open if I have further problems.