Issue: Permanent Stat Item Did Not Drop After Defeating Boss

I defeated the boss, but my character died at the same time. As a result, the item that was supposed to drop did not appear. I also tried resetting and defeating the boss again, but the item still did not drop.
Could you please advise how this issue can be resolved?
Thank you for your support.
IGN : มันจะเก่งไหมเนี่ย
Player ID : KluyPJ#9737