Mobs like Villagers and Piglins will still take fall damage even if they are wearing Rainbounce Boots.
/summon minecraft:villager ~ ~ ~ {ArmorItems:[{id:"alexscaves:rainbounce_boots", Count: 1b, tag: {Damage: 0}}]}

Also the Rainbounce Boots give you the damage effect when landing sometimes. So when you land, you hear the damage noise. Not landing, but damage.

Is that intentional? You don't take damage but it's definitely weird to occasionally hear it.