Powdered snow not melting when next to water.

This is because the water doesn't flow to the powder snow. Water only destroys powder snow when flowing into it, not when simply touching it. If there is a hight difference near the water where it goes down in a maximum distance of 5 blocks from the source (For example a hole), the water will just flow in that direction instead of flowing in all directions.

That explanation is correct. Water only destroys powder snow when flowing into it. It doesn't in this case.