Unknownreported this bug
Tests were made on R1.20.1 using MythicMobs 5.8.2 Release and ModelEngine 4.0.8 Release and R1.21.3 using MythicMobs 5.9.0 Build 5405 and ModelEngine 4.0.9 Build 2134.
With some chance upon spawning a mob with modelengine model it doesnt get properly unloaded when mob is removed (logs with warns [Legacy attachment omitted]). Mob itself has an algorithm of applying model, switching 16 bones and despawning after that (mob file and model file [Legacy attachment omitted] [Legacy attachment omitted] [Legacy attachment omitted]). Video of spawning modeled mobs and encountering an issue [Legacy attachment omitted]. Error is persistent through different versions of minecraft and plugins.