With stackableShulkerBoxes set to true or 16 there appears to be a duplication method. By holding a stack of 2 or more colored shulkers and cleaning them in a cauldron, you receive an uncolored shulker box but the colored one isn't removed. I tested this on an otherwise unmodified version of the game, with only carpet (and fabric api) installed, in survival and creative. No other game rules are enabled. I tested this in 1.21. I assume this may be an effect of (1.20.6?) change of how the gamerule stackableShulkerBoxes works.
To reproduce: 1. Create a new creative world with carpet. 2. Set stackableShulkerBoxes to true or 16. 3. Grab a cauldron, water bucket and at least 2 shulker boxes of any color. 4. Place the cauldron, fill it with water and attempt to clean the shulker box. 5. Observe how you receive uncolored shulker while colored are not consumed.

I hope this report has been comprehensive enough.