When you kill tropical fish, cod, salmon, and pufferfish, their dead body will not get pushed away when killed. Swords with the Knockback enchantment, End crystals, and TNT do not stop the bug from occurring. This issue is caused by MCPE-19222

Steps to Reproduce:

1. Find any fish mob such as tropical fish, cod, salmon, etc.

2. Obtain a sword with the Knockback enchantment.

3. Kill the fish.

Observed Result:

Notice how the fish doesn't take knockback on death.

Expected Result:

The fish should take knockback when it gets killed.

Notes:

It looks like the Minecraft developers has unintentionally set "minecraft:physics, has_gravity" to false, which is main reason why tropical fish, cod, salmon, and pufferfish don't take knockback upon death. I'd suggest setting minecraft:physics, has_gravity to true, which would give them the intended behavior on death.