The secret to chaining divisions to brigades to battalions in the OOB display is that the units must belong to the same team (country) in which they are placed, i.e., if they are sitting on South Korean soil then they must belong to South Korea. Otherwise, they will only aggregate into a Division and will not be expandable in the OOB.
And here is the proof for a campaign I've been messing around with in the Iranian theater:
