TNT can explode underwater in vanilla Minecraft

Description

Normally, TNT cannot destroy blocks underwater. But if you put a block above that’s affected by gravity (e.g. sand or gravel), it can destroy blocks.

Steps to reproduce

Place a block above the TNT that’s affected by gravity and detonate the TNT.

Expected result

The TNT only deals damage.

Actual result

The TNT explodes, deals damage and destroys blocks.