OP here. It was... I was really fortunate that some other company had figured it out in parallel with MS, because I was hitting a dead end. I got to the point

OP here. It was... I was really fortunate that some other company had figured it out in parallel with MS, because I was hitting a dead end. I got to the point where I was pretty sure that WPAD wasn't working right, and was in the midst of capturing all sorts of data and escalating that, when a side conversation prompted someone to ask "was it you I worked with on X?". Then it all came together. I think this is one of those things that ends up being so esoteric most folks haven't worked on it, then the chain of events leading to it... But the moment I heard the trigger I was able to repro it... That was quite a relief.