Scripts with different sync behavior modes starts not in the same frame. That makes it impossible to control the order of start in combination with the DefaultExecutionOrder attribute.
Example:
[UdonBehaviourSyncMode(UdonSharp.BehaviourSyncMode.None)]
public class BehaviourSyncModeNone : UdonSharpBehaviour {
}
starts before:
[UdonBehaviourSyncMode(UdonSharp.BehaviourSyncMode.Manual)]
[DefaultExecutionOrder(-1000000)]
public class BehaviourSyncModeManual : UdonSharpBehaviour {
}
World to reproduce (Bug Booth 1):