The mangrove leaves can be obtained directly without Silk Touch by simply breaking them by hand. This happens only by chance.

How to Reproduce

1. Find a mangrove swamp.
2. Break a lot of leaves with your hand.
3. Check if you get mangrove leaves.

Observed Result
Mangrove leaves can drop itself without Silk Touch.

Expected Result
Mangrove leaves should never drop itself unless using Silk Touch tools.