unity – Why OnTriggerExit is raised when object didn’t exit collider?

Hoping you can spare some time.

To elaborate: the scenario is, I have a "spawned object", a "box collider", and a "ground".

"spawned object" - has collider, trigger not checked, has rigidbody
"box collider" - trigger checked
"ground" - trigger not checked, has rigidbody

Now, the "spawned object" is instantiated inside the "box collider", then falls and hits the "ground", bounces twice then stops.

Upon spawning "spawned object" – it’s set to isKinematic = false, then it’s isKinematic set to true when bouncing stops.

What happens is, “box collider’s” OnTriggerExit is triggered after setting “spawned object’s” isKinematic to true it seems.

This isn’t the behaviour I’m expecting. I’m hoping you can shed some light on what’s happening. And what can I tweak so that OnTriggerExit is triggered only when “box collider” left the “spawned object”?

Please let me know if it’s unclear.

Thanks in advance.