The bug

The DeathTime tag represents how long the death animation is shown already and removes the entity if the value of the tag reaches 20. However, if you change the DeathTime to something different than 0, client-side the animation will still last 20 ticks.

How to reproduce

1. Use the following command

   /summon villager ~ ~ ~ {Health:0f,DeathTime:-41s}
   

2. Reopen the world after the death animation stopped (or 2 seconds)
   → The dying villager is still there
3. Summon the villager again and close and reopen the world after about 4 seconds
   → The dying villager is gone because the value of the DeathTime tag reached 20 after about 3 seconds

The reason

The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.

This happens very likely because the DeathTime tag is not tracked by the data manager.