The bug
If a mob such as a zombie is in powder snow, it does not catch fire from the sun. This is inconsistent with other methods of setting mobs on fire while they're in powder snow, such as fire charges.
Steps to reproduce
Prepare an area of powder snow
Spawn a zombie in the middle
→ ❌ The zombie does not catch fire from the sun
Expected behaviour
The zombie will be set on fire from the sun. This will lead to it being extinguished by the powder snow, therefore removing the powder snow that the zombie is standing in.
Attachments
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In my testing, I found that summoning a mob in powdered snow will allow it to not catch fire and the powdered snow block will stay, however a burning mob that walks into powdered snow will immediately get extinguished and break the powdered snow, as expected.
(21w15a)