Caused by the fix of MC-143687

The bug

When you go to the top of a scaffolding tower such that it's not supported by any other solid blocks and you look down on the top face and place begin placing scaffolding, it extends out horizontally from the scaffold you're aiming at. In the previous snapshots, when the scaffolding hit the limit, it places the 7th block which will then fall, very useful for creating platforms rapidly without having to climb up and down. However it will not place the 7th block and indeed even if you aim a scaffold at the area and shift right click to place it it will not place.