My first sight of 'dying entities' was in a hole full of witchs, skeletons and evokers (I made it for fun at first) , where serveral skeletons were killed by evokers while healed by witch's instant damage potions in the same time, so these skeletons rarely gained a state of 'alive and dead'. These 'dying entities' cannot be killed except killed by negative effect potions.

How to produce manually:

I chose pillagers for the test

1. Place a command block at a height that can kill pillagers.

2. Type summom pillager ~ ~-2 ~ in the command block.

3. Active the command block Intermittently so that it will keeping summon pillagers from bottom of it.

4. Keep in throwing healing potions at where they fall to death.

5. Serveral dead pillagers will come back to life.

I hope I will never meet them in survival mode XD