This is not a bug.
Your items are still there, they just immediately stopped being rendered after the death screen popped up. This will happen anytime you die in singeplayer and upon any block.
If you return to your death location within 5 minutes of the chunks containing your items, you will see that they are there.
Some things:
Which direction the player moves towards prioritizes east, then south, then west, then north. (If a cushion has four holes around it, it will go to the eastern one. If the eastern one is blocked but the other three remain, it pushes the player southwest, etc.) The only time this is not true is when there is a hole beside all sides of the cushion except the western side: it pushes the player northeast when, according to the pattern, it should push the player southeast. Note: the prioritization can be altered by summoning a cushion with commands that is not at the exact center of a block. However, whatever prioritization that is chosen remains consistent.
I have not noticed the rotation of the placed cushion or the player to have any impact.
Depth of hole does not matter
Bug happens only when the cushion is placed
Curiously, the bug will happen with the cushion placed on seemingly all full blocks EXCEPT a stack of 8 layers of snow. The bug will still happen on a regular snow block. The bug will also still happen if the cushion is placed on two of the same slabs stacked upon one another.
The bug will also happen if the cushion is placed upon a layer of snow, pressure plate, repeater, or anything else that places the cushion’s y-axis below 0.35 blocks above the ground. The highest I could get by summoning a cushion with commands and still have the bug happen was +0.3499998999999966997620504116 above the base level.
my guess is since the cushion is an entity, the player and the entity collide when the player dismounts from the cushion and the player gets pushed as a result. but what do i know :)
curious that it doesn’t happen with the snow layers though, and only when there is a hole next to the cushion!