The bug
When you place a scaffold on the floor in water then try to place again in the same spot, it won't stack up like it does when in the air.
MC-139018
Scaffolding doesn't stack from the bottom under water
Resolved
Fixed
12
csg
Confirmed
scaffolding, water
Minecraft 18w45a, Minecraft 18w46a, Minecraft 18w48a, Minecraft 18w48b, Minecraft 18w49a, Minecraft 18w50a
Minecraft 19w03a
Retrieved
Confirmed in Minecraft 18w46a. It is not solved.
Now the problem is worse. Under water, before (18w45a) you could manually put a scaffolding block on another scaffolding block, although you can not stack it from below (reason for this bug report). Now, you can only put the first scaffolding block, you can not manually put any other scaffolding block on top (it fall only), and you can not put a second scaffolding block in flowing water either.