Steps to reproduce:
1.Open attached “RuntimeAddScriptTrigger.zip” Unity project
2.Open scene “Test"
3.Enter Play mode
4.Expand “Parent” object in the Hierarchy to see its child objects
5.Attach the “Triggers.cs” script to “TriggerObject”

Expected results:
OnTriggerStay is called and debug message “Stay triggered by BoxCollider" is shown
Actual results:
OnTriggerStay is not called

Reproduced in: 2017.3.0b3, 2017.2.0f3, 2017.1.2f1, 5.6.4f1, 5.5.5f1, 5.5.0f1

Note: Works as expected if the script is attached while not in runtime or parent object does not have a Rigidbody component.
_____________
By design:
OnTriggerStay depends on having processed OnTriggerEnter first internally, which is not done if there were no scripts interested in that event at the time the overlap was detected first (for performance reasons)