Order going to free flow

Hello Experts,

I have an issue wherein an order is getting confirmed from a plant where there is less stock. The order is going through the rule based atp in apo.

Order qty-100

Stock in Plant 1-80

Stock in Plant 2-80

Rule set up in apo-

Fan based structure:

Plant1--Plant2

            Plant3

Expectation is that the first 80 should be fulfilled by plant 1 and rest 20 should be given by plant 2.

On investigation I found that in the line item in the order, one of the field :ATP Quantity is 999.999.999.999.999

I believe that the system is ignoring the actual stock and going to a free flow.

So my question is where does this qty comes from. What influences this and how can I correct it.

Thanks in advance

Karan