MCPE-19222 has not been fully fixed; some entities that don't walk aren't knocked back when killed, plus status effects and explosions that kill entities don't knock any entity back.
The bug
Entities killed by a status effect (wither and instant damage, but might include more) or explosions have their coordinates locked when killed, and are not knocked back.
Can confirm. When they get killed by tnt, it works fine, but when killed by ender crystal, they don’t get knocked back.