After using an elytra to land on a block or slab that does not have another block(s) below it, the player takes damage multiple times as if they where falling through but will stay on the block. After a while or instead of taking damage, the player will fall through the block or slab. After getting on top of the block or slab by climbing scaffolding or using blocks, the player will not take damage. It seems to only happen after landing with an elytra.