Cannot create water source blocks on top of scaffolding

Updated description by @unknown

Steps to reproduce

1. Place 3 scaffolding blocks in a row.

2. Place a water source block on top of each of the 2 scaffolding blocks on the ends.

Expected result

A water source block is created on top of the middle scaffolding block

Actual result

Flowing water fills the space on top of the middle scaffolding block, but it does not convert to water source block.

Original description

Water will not turn in to water sources when you put water sources on the edge of a square (at least 5x5 or bigger) with a scaffolding floor