Shulkers can teleport when taking damage, even if they had full health (which is more than half).

See the movie for detail.  This may relate to other strange behaviours of shulkers that teleported on their own.  For example, shulkers shoot bullets while closed  (reported in MC-216251 ), which is confirmed in my movie, too.

update:

To reproduce this, you may have to make shulker duplicate by a shulker bullet.

I tested with a sword enchanted with Sharpness Lv 25 (20 damage), so shulkers with less than half health can't survive.

https://drive.google.com/file/d/1Inof-kWlg7e6g1qtGcQnjKgHYfokrhby/view?usp=drive_link

https://drive.google.com/file/d/1zfukVmjQobzUnzxA5ccnWcSgdk1M2vsN/view?usp=drive_link

I also attached a world data to test this behavior. The testing farm is located at (X, Z) = (-50, -500).