The bug

Shulker copies can be created with shulker bullets, even if the bullet did not come from a shulker. I'm not sure if this is intended. From the 20w45a changelog: A shulker hitting a shulker with a shulker bullet can make a new shulker.

To reproduce

1. Summon a shulker.
2. Run the command

/execute at @e[type=shulker] run summon minecraft:shulker_bullet ~ ~2 ~

multiple times. If the shulker dies, spawn a new one.
3. Eventually a new shulker will be spawned.

Alternative steps
1. Summon a shulker.
2. Summon any mob that is not a shulker with the UUID [I; 0, 0, 0, 1]. For example, a cat:

/summon minecraft:cat ~ ~ ~ {UUID:[I; 0, 0, 0, 1]}

3. Run the command

/execute at @e[type=shulker] run summon minecraft:shulker_bullet ~ ~2 ~ {Owner:[I; 0, 0, 0, 1]}

multiple times. If the shulker dies, spawn a new one.
4. Eventually a new shulker will be spawned.

Note that this alternate method is currently unreliable due to MC-226344.