David Simmers

I just tried it in 1.16-pre2 and it happens in the conditions I laid out.  If the player is exactly 0.3 away from the Shulker box in both X and Z then you still get caught on the corner of the hit box.  So if you place a Shulker box at 10, 10 and then are at exactly 9.7, 9.7 then it will still happen.  The box lifts you up and then you fall off it, playing a "soft fall" sound.  If you close the box mid-opening then you fall and take 2 hearts of damage.

Because spawning from a bed is fixed, I can't find any other way to get to EXACTLY .7 .7 during normal game play.  If you place 2 blocks and try to align yourself to them you don't end up at exactly .7 .7 even though the debug screen shows it.  You end up at .699999988079701 (which you can see by using the TP ~ ~ ~ command I guess?)

So the fact that you can clip into the corner of a shulker box hit box and take fall damage from falling off it is still a bug, however there does not seem to be a way to achieve that other than using the TP command.

There is some additional behavior that goes along with this.  If you use minecarts for entity cramming then they will roll back and forth even after all the mobs die.  The only way to get the noise to stop is to turn the sound down to 0% and then back on.  It would be really lousy to fix that part as a bug and then leave the minecarts endlessly making noise.

Here's a Youtube video I made where you take the fall damage.  The exact moment you cancel opening doesn't seem important, just that you close the shulker box before it completely opens.  If you let it completely open you will fall and it will play that strange softer noise but you don't take any fall damage.

I've figured out how to 100% reliably recreate this in 1.15.2

Place Shulker Box at 0 ~ 0

TP to 1.3 ~ 1.3

Open Shulker Box

As far as I can tell it works on all 4 sides as long as you are exactly .3 away (so on the other side of the box it would be at coordinate .7)  

I do have some additional bugginess related to this.  First is that it plays a noise similar to the fall damage noise but that isn't quite the same.  Its like a softer version of the fall damage noise.  Which is extra strange because you are only falling 1.5 blocks.  What is even stranger is if you close out of the shulker box BEFORE it finishes the opening animation you actually take fall damage and the regular falling noise plays.