sometime mobs despawned after death

Need more info, reopen with related mob configs

서리좀비:
  Display: '&f서리좀비'
  Type: DROWNED
  Faction: monster
  Health: 100
  Damage: 12
  Drops:
  - 3잡몹 1 1
  Options:
    PreventJockeyMounts: true
    NoDamageTicks: 5
    Despawn: false
    AlwaysShowName: false
    MovementSpeed: 0.35
    PreventOtherDrops: true
    PreventSunburn: true
    PreventRandomEquipment: true
  Skills:
  - skill{s=버프밟판_소환} @self ~onDeath 0.1
  - skill{skill=서리손길} @target ~onAttack 0.1
  AIGoalSelectors:
  - 0 randomstroll
  - 1 meleeattack
  AITargetSelectors:
  - 0 randomstroll
  - 1 players

This happens with most monsters, not just these monsters. Could mythicdungeon's instance world be a problem?

If it's inside of a MythicDungeon instance that could be a problem yeah, MM flags a mob to not be despawned by spigot but that wont stop other plugins from removing them

But this problems after update 5.5.1 to 5.6.2. I noticed this problem and rollbacked the version back to 5.5.1, and the problem disappeared. this problem caused by update mythicmobs.

this issues will not be fixed? i can't update mythicmobs version due to this issues.

Closing due to inactivity and major version changes since posting. If this issue is still present, re-open this issue with a modern-version server log, configs used to test and steps to reproduce.
Since this issue is also related to MythicDungeons, reach out in our Discord support channels for troubleshooting MythicDungeon setup and updates related to this.