With Consumable Inventory relation between two inventory organizations and demand exceeding supply, shortage of the consuming Inventory organization would not be published in the Parent Inventory Organization.

Answer

Scenario -
Have a Inventory organization as Ent1. Have another inventory organization Child1 a child of Ent1.
Have another child of Ent1 as Child2 and this is not an inventory organization and hence totally dependent on Ent1 for inventory as Ent1 is the inventory organization for Child2.

Solution -
GetATP API would only project the inventory picture and if anything is available to promise in the existing supply.
Ent1 is an inventory organization which has a node N1 with 20 quantity to fulfill the demands.
Child1 is a child of Ent1 but it is an inventory organization and consumes the inventory of Ent1.
Child2 is another child of Ent1 and its inventory organization is Ent1.

Now lets place our 1st order of 10 quantity from Child1.
At this point my inventory picture would be :

Supply - 20
Demand - 10 (Child1 = 10)
Available to promise - 10

Now lets place our 2nd order of 8 quantity from Child2.
At this point my inventory picture would be :

Supply - 20
Demand - 18 (Child2 = 8, Child1 = 10)
Available to promise is 2

Now if we place our 3rd order as 5 quantity from Child1.
At this point my inventory picture would be :

Supply - 20
Demand - 20 (Child2 = 8, Child1 = 12(i.e 12=10+2, 3 shortage))
Available to promise is 0
Shortage = 0

As the shortage has occurred at Child1, Ent1 would not hold itself responsible to fulfill it as Child1 by itself is an inventory organization.
And among the 15 only 12 demand can be fulfilled by Ent1 and remaining would be assumed to be fulfilled by Child1 (as it calls itself as an inventory organization)

If, In case, Child1 was not an inventory organization and was fully dependent on Ent1, surely Ent1 would take care of all demands of Child1 and would hold itself(Ent1) responsible for any shortage occurred at Child1 as it is completely dependent on Ent1.

Hence the behavior of the getATP API is working as per product design.