Follow On is Session and Account Id Dependent

 Scenario 1:
Order O1 having line items L1 and L2
Follow on order F1 is created on L1
Now in the same session we navigate to a different order O2 for the same customer having line item L3
We create a Follow on order on L3,
Now instead of creating a new follow on order with line item L3, its appending the line item L3 to the original follow on order F1

Scenario 2:
Order O1 has line item L1 and L2
Follow on order F1 created on L1
In the same session we navigate to different order O2 for different customer having line item L3