Whenever a player attempts to jump/fly onto a 1.5 block gap (1 full block and a slab of air) the player won't be allowed to pass through/rest on the ledge.
MC-147410 Jump+sneak
MC-150938 Sneaking in a 1.5 height gap doesn't work if you have to jump into the gap
MC-255650 Can't jump and move forward with slab above head whilt crouching