Unknownreported this bug
Summary
It seems that passive mobs ignore the despawn option completely, along with the despawn default option in the config.yml. A parrot and turtle both had the nbt value for PersistenceRequired at 1b with despawn set to false, while an evoker had it set to 0b while set to false. The wiki lists that the relevant nbt tag for despawn is PersistenceRequired.
Steps to reproduce
[Legacy attachment omitted]
Current behavior
Some mobs set to despawn are not despawning
Intended correct behavior
Mobs set to despawn should despawn.
Server log file
Debug log snippet
Proposed fixes
Minecraft may be setting the PersistenceRequired tag to true after mythicmobs affects the tag. If so, just set the tag later in the spawn process.