Queue._getBuildableItems looks at queued items and says they are buildable on a Computer if the task is not tied to a label or label matches the Computer. If the task is not tied to a label it does NOT check if the Computer is Mode.EXCLUSIVE (ie, "for tied jobs only").
Thus, an on-demand slave may launch its connection when an untied job is in the queue, even though that slave cannot run the task.
_getBuildableItems should either check the Mode, or maybe better use Node.canTake(task) to determine if the task can run on this Computer.