One change made in 21w05a to powder snow was that they were given the ability to extinguish flaming arrows. However, if a flaming arrow is shot through a powder snow block diagonally, this sometimes doesn't happen
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Can confirm in 21w05b.
Can confirm in 21w06a.

Seems to be (unintentionally?) fixed in 25w02a due to the fix of MC-7697. I couldn't reproduce
Edit: Nevermind, when shooting at the very very corner of the block it still sometimes does not register the arrow. I find it harder to reproduce in the new version however, it seems it happens less.
Can confirm. If they are on the edge of the powder snow, they don’t seem to get extinguished.