I'd think that if the whole section were giving way, but it's only the very top layer of the wall. When a new block gives way, it only takes out the blocks immediately next to it, so it's spreading horizontally more than anything. It's also affecting the catwalk, where if you attach the new catwalk around the outside at the section that's causing the issue (immediately crumbling upon block placement) even if the unsupported span is only 1 or 2 spaces, the catwalk will crumble and fall the same way the wall sections do. It's almost like the gravity or block weight is screwed up in a very small area.
Like I said, not sure if it's a bug or not, but check it out when you get a minute. It's VERY strange behavior and might be an artifact of 14.7, since I've never seen anything like this before... at least not without there being some major lack of support and that would take down entire sections all the way to ground level.