I first observed the issue in multiplayer 1.16.1 but it is the same in singleplayer. When placing a shulker box under my feet in a 3 block high room so my head is directly against the ceiling and open it, the player falls inside of the box and stays there. Upon closing the inventory and therefore the box closing the player returns to normal.
To reproduce, simply replicate the setup shown in the video clip below.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-148097, which is a report that has not been resolved yet.
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