sometimes mobs are not spawned if the same instance dungeon is created.
i wrote this issues before 1 month, but you didn't replied.
im tested serveral times. i turn on mythicmobs debug mode. (to figure out there's no problem with the creation of monsters) youtube : https://youtu.be/F_Exeh9_6r4 (50 seconds)
It is certain that the monster does not spawned. (In video in 48 seconds, distance triggers must be activated when player enter the portal) i checked console debug messages, but MythicMobSpawnEvent for monster is not It was not printed on the console at that time. As I said before, this problems does not always occur.
and many times bug report from users (over 30 times) i found some in common. This happens only when multiple identical instance dungeons are created.
please check this issue.