PC Slime King don't despawn

[Spurkit]

Steam or GOG

Steam

Single Player/Multiplayer

Single

Operating System

Windows 10

Terraria Version

1.4.0

Controls Used

Keyboard/Mouse

So, in Slime Rain, I'm slayed enough slime to summon Slime King. But in fight I was slayed, but Boss don't desappear. Affter respawn I killed him (I'm not proud of it, but this is the master mode).
So it's feature or nuh?