nether boss from fortune teller quest doesn't disappear after defeating it

fortune teller gave me an objective “Defeat the boss at nether realm depth 145”, though my farthest nether realm is about 220

the boss (Deathwalker) was there, but after defeating it I could keep fighting it again and again and I wasn’t able to open the chests

Duplicate of

oh, sorry I just did ctrl+f for boss to see if anyone made it (and someohw missed it still)