This is something I keep a reasonably close eye on, as both of my children are dual US-Japanese citizens.

Both countries require the choice when the child becomes a legal adult, but neither seeks to actively enforce it (unless there is a reason, such as pursuing a security clearance in the US).

It’s an odd area of law.

EDIT: The statement that they both require the choice was pointed out to me as incorrect. I lived in the classified world for decades, where that choice IS required and incorrectly generalized that.