Found the problem.
I have to explicitly create an instance of BehaviorManager and not let it be auto-created, and then ensure it doesn't get destroyed.
It seemed to get auto-created in scene A.
So, when scene A got unloaded, the BehaviorManager got unloaded, too.
Does the BehaviorTree runtime link itself to a specific scene? I am experiencing a weird bug.
Load Scene A additively
Spawn BehaviorTree in Scene A
BehaviorTree works fine
Load Scene B additively
SceneManager.MoveGameObjectToScene(gObj, sceneB)
Unload Scene A
BehaviorTree just... stops...