So I found a solution to this and it involved a different approach. This thread was the inspiration:
Rather than using a Timeline to trigger the material transition, I use a distance-based falloff. It works beautifully and gives me better and more intuitive control over the falloff range.
Also, the TriggerBox is not required any more, since I’m not using any overlap events. Any object can be used as the trigger, because all we need is the distance between the source and target objects.
Here’s the blueprint:
And here’s a demo video of it in action. I made the trigger box visible. It is animated using Matinee.