When falling from any height with frost walker boots equipped, there is a possibility for ice to generate when falling. This can cause sudden death if the player falls from a high enough height.
Frost walker only works when walking, meaning you have to move from a block of the same level to create the ice, falling, flying, teleporting, riding an entity, should not create ice.
1. Equip frost walker enchanted boots of any level.
2. Fall from any height, (more noticeable from high heights.)
3. Land atop a flat water surface while holding any movement button.
4. Observe that ice is created and you take fall damage.
The Official Minecraft Wiki also states it is not an intended mechanic as well.