I’ve done same setup as yours and it works :
I think your cast is wrong. Your light BP is called “MyBPLight” as I can see on the content browser, and I guess it is the class assigned to the child component. You must cast to this class, so you should have a “cast to MyBPLight” instead of a “cast to point light”.
Then grab the light component of this class, it should be named “point light” if you haven’t renamed it, and not simply " light component.
However I couldn’t explain why it works on begin play…
Edit : I advise you to alway have same prefixes for better clarity, “BP_NameOfTheBlueprint” for example, this way when you use cast node you directly know if you are casting to a blueprint class instead of casting to a component