Looting 3 Swords Give Same Drops As Non Looting 3 Swords

The vanilla loot tables for both 1.15.1 and 20w21a include:

{
          "condition": "minecraft:random_chance_with_looting",
          "chance": 0.5,
          "looting_multiplier": 0.0625
        }

If they aren't affected by looting in-game, then that's your bug.

Can replicate in 20w21a. Shulkers consistently drop 0-1 shulker shells with looting 3.

I think it is intentional that shulkers drop 0-1 shells. The Looting enchantment only increases the chance of a killed shulker to drop one.

Shulker Shell drop amount is not modified by Looting, drop chance is.

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It's not an issue, it's intended by the developers.