Dropping the last target without the special lit scores only 5000, steps up the bonus, and resets the drops. It does not score 500, then 5000, advance the bonus, then reset. Only 5000 at sequence completed.

I've never had that fault happen but it sounds to me like a drop target switch is gapped too close and that is causing it. The outlanes also score 500 and bonus advance when not lit, so that is also a place to look. I don't think it's any of the B I G rollovers because the lamp would go out in that case and you'd notice it.