If you fly downwards onto a layer of water, lava, slime blocks, hay bales, or beds, the damage reducing properties of these blocks are completely ignored.

To reproduce, find or create a one block deep pool of water and set the weather to rain, then dive straight down into the pool and take 0 damage (unless you "fall out of the water" and experience MC-132965). When you try the same thing using the elytra, you will die as if you fell on a block and never touched the water.