The bug
When a player jumps into an opening shulker box, are moved horizontally by the shulker box and stay floating until the opening animation of the shulker box ends.
To reproduce
Place a shulker box on the side of a block, so that it is at eye level with the player.
Jump, and while midair, open the shulker box.
→ ❌ The player does not fall while the box is being opened.When the box finishes opening, the player falls.
Video
Here is a video link of the bug being recreated in the 20w17a:
That is not a bug. It is a feature of the game