When a mob dies, it will usually fall down to the ground while in their death animation, but a few mobs seemingly have different velocity values while in their death animation.

One example of this is to do with the allay, bee, and the parrot. When they are killed, they start to just keep ascending upwards and never seem to stop. To replicate this, use one of the commands I have listed below and kill the mob while it is moving upwards, or else nothing happens.

/summon parrot ~ ~ ~ {Health:1b,DeathTime:-200s}

/summon bee ~ ~ ~ {Health:1b,DeathTime:-200s}

/summon allay ~ ~ ~ {Health:1b,DeathTime:-200s,Attributes:[

{Name:"generic.max_health",Base:1f}

]}

You will see that they will just keep on moving upwards for some reason. There may be more mobs that have this behavior, but these are the only three mobs I know about.

Another example of this is to do with the strider. When a strider is in its death animation, its velocity just comes to a halt for some reason. To replicate this, use the following command to summon a strider with slowfalling and kill it while its in the air. You will see that the strider seems to just hover in place instead of continuing to fall to the ground.

/summon minecraft:strider ~ ~ ~ {Health:1,DeathTime:-200s,ActiveEffects:[

{Id:28,Duration:999999,Amplifier:0,ShowParticles:0b}

]}