Mac OS 10.14.6
Water doesn't create new water source blocks on top of dry scaffolding even if two adjacent water sources blocks flow in the same space.
Steps to reproduce:
Make small rectangular 2 block deep hole, fill bottom layer with scaffolding or other blocks to test.
Apply water or ice to two adjacent edges of top layer (don't waterlog bottom layer)
See if top layer spreads with water source blocks or just flowing water.
With dry, but waterloggable blocks such as slabs the top layer of water spreads.
With dry scaffolding, the top layer of water doesn't spread, and there is just flowing water.
With waterlogged scaffolding, the top of layer of water spreads.
A layer of scaffolding should allow water to spread over it like other waterloggable blocks.
Before picture shows experimental setup. Please note the glass blocks show waterlogged status of blocks on the bottom layer.
After picture shows how the water reacted to layer below it.
I was trying to make an ice farm with scaffolding and noticed that water didn't spread on top of scaffolding (needed to flood the platform with water source blocks and the water flowed, not spread)