How to reproduce
Summon a snowman at y=67 with a block below him at y=66
→ ✔ The snowman places a snow layer at his feetSummon a snowman at y=66 with a block below him at y=65
→ ❌ The snowman doesn't place a snow layer at his feet
Note: This only occurs in a plains biome
The ticket that was marked as 'duplicates' talks about snow layers not being placed in certain biomes. Neither the wiki or that bug report mention a height limitation. Thus this bug should be reopened as according to the wiki snowmen shouldn't leave a trail in plains, while they do above y=66