Scaffolding + left click silently consumes scaffolding extended above world height limit.
I had a stack of 64, was /very/ much above 64. I just left clicked the whole stack to extend the height up... and when I got to the top of the tower it was 129.62 (128 for the top of the last block [0::128] = 128 blocks + 1.62 for player head level).
Placement of the scaffolding should in all cases check that the scaffolding can in fact be placed on the top block.