Summary:
Powder snow can be placed in the same space as non-full blocks.

Steps to Reproduce:

1. Place a non-full block (e.g a fence);
2. Place a powder snow block in the same space as the fence, while pointing at an adjacent block.

Observed Results:
Powder snow can be placed in the same space as the fence.

Expected Results:
Powder snow should not be able to be placed in the same space as non-full blocks.

Video:
Minecraft 2020-12-17 14-46-13.mp4

Blocks that can coexist with Powder Snow:

• Slabs
• Stairs
• Fences
• Fence gates
• Walls
• Signs
• Item frames
• Paintings
• Doors
• Trapdoors
• Iron bars
• Glass panes
• Ladders
• Brewing stands
• Vines
• Redstone repeaters
• Redstone comparators
• Levers
• Lanterns
• Daylight sensors
• Hoppers
• Flower pots
• Pressure plates
• Banners
• Bells
• Enchantment tables
• End portal frames
• Rails
• Campfires
• Carpets
• Cacti
• Chests
• Buttons
• Scaffolding
• Tripwire hooks
• End rods
• Conduits
• Mob heads

Blocks that get destroyed by Powder Snow:

• Twisting vines
• Weeping vines
• Flowers
• Torches
• Sweet berry bushes
• Crops

Notes: This behavior closely mimics waterlogging, every block gets affected in the same way.