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Triggering a trigger volume with a Capsule parented to a camera

On Answer Hub there is a question today about triggering an event using an object attached to a camera. The idea (if I understand right) is that an object is attached to a camera, and when the camera moves at some point the object enters a trigger volume and the event fires. Trying this myself with a Capsule held out in front of a camera, I’ve notice something odd. There is clearly something I don’t understand about how collision detection / triggering works and I’m hoping someone out there can explain it to me.

Scenario:

Basic 3rd person starter content.
An additional capsule parented to the FollowCamera and positioned way out in front. (See image)
A trigger volume is added to the scene. The idea is that when the camera moves, the extra capsule parented to the camera enters/leaves the trigger volume and causes some text to change color.

Observation:

At first I thought this was working as I expected, but I’ve noticed something strange. If the character is moving forwards/backwards, the text changes colour appropriately whenever the additional capsule enters/leaves the trigger volume. However, if the character is standing still, moving the mouse rotates the camera and moves the capsule parented to the camera in/out of the trigger volume (I can see the capsule moving), but nothing gets triggered. No event gets triggered until I press forwards/backwards on the keyboard; as soon as I press the move forward/backward key the color updates dependent on the current position of the capsule relative to the trigger volume (i.e. whether the capsule is in/out of the trigger volume when the button is pressed, not whether pressing the button moves the capsule into the trigger volume).

In short - it seems that it is not just whether a capsule enters/leaves a trigger volume that matters as far as triggering is concerned. Some kind of additional input seems to be needed to get the engine to check whether collision is occurring between a capsule and a trigger volume. Can anyone explain what is happening?