The bug

Area effect clouds (created by lingering potions) apply their effect to dying mobs.

This might be intended since the mob is still in the world and can be seen but on the other hand interaction with a dying mob is limited and therefor this can be seen as bug as well.

How to reproduce

1. Have entity hitboxes be shown (press F3 + B)
2. Summon a dying villager in an area effect cloud

   /summon villager ~7 ~ ~ {Health:0f,DeathTime:-280s,Passengers:[{id:"area_effect_cloud",Radius:4f,Duration:1200,RadiusOnUse:-0.5f,Potion:"healing",ReapplicationDelay:10}]}
   

   → You will notice that the area effect cloud applies the effect and therefore shrinks