Why doesn't population death yield DNA points?

Some DNA points are automatic, in that you don't have to click any bubbles to get them. Normally I see two big streams of these points.

1. On the way up, as the plague spreads.
2. On the way back down, as the population dies off.

Without too much trouble, I unlocked all of the plague types on Normal difficulty. Now I've gone back to Bacteria on Normal to try for a higher score.

But for some reason I don't get that second stream of DNA points after the entire population is infected and I switch my plague into a lethal mode. Even after a couple of billion people have tragically succumbed to my creation, there are no automatic DNA points... just 2 or 3 orange bubbles to pop. I've seen this on 5-6 games now. I tried playing on Casual again and got the expected second stream of points.

What's gone wrong? Can this behavior result from genetic code choices or symptom combos? Or does Normal difficulty just get harder once you've unlocked all the plague types?

UPDATE

After seeing this problem in at least half a dozen games, it's stopped happening. Despite using the same basic strategy, now there is a second stream of automatic DNA points. In all cases, my strategy creates a high Severity level and moderate Lethality level.

I'll leave this question active for a while. Hopefully someone can explain why you sometimes get a pile of automatic DNA points in the "death phase", and sometimes literally zero automatic points, despite having decent Severity and Lethality.