"How is the night Rehema?"
Translation:Habari za usiku Rehema?
I think the reason is that in English, the evening is often called night, and so if there's a reference to night, it could in principle be either jioni or usiku. That said, earlier on it seemed like they were making the distinction in English, as well, to make it more clear how the distinction was in Swahili, so you would expect the same to have been done here. Given that, it might just be an oversight... I'm not sure.
In this case, I believe the use of ya or za with Habari depends on the country. Ya /za should agree with the preceding noun in this case habari (news). Habari is an N class noun. Therefore, za should be used grammatically speaking since ya is for singular N class nouns and za is for plurals.
However, in Kenya, this rule is not followed. In Kenya, they say habari ya jioni, mchana, asubuhi, etc. I am told that in Tanzania the grammatically correct za is used. But, if you use za in Kenya they will understand what you mean. You will just sound like a tourist, which is fine.
Some people explain that za is used for general politeness and ya is used when you really want to know what's going on. See website below. http://www.learnswahili.net/swahilicourse/learnswahilicourse_lesson2.html
And, perhaps this is why even Duolingo uses ya and za at different times. What is the news of the day? (General) vs. What's your news of the day. (Personal)
So maybe Kenyans choose to be less formal and that's why it is commonly used in Kenya. I don't know. I just know Kenyans say Ya even though it should be za if you are following the rule.
One disclaimer: It could also depend on where in Kenya. I stayed primarily in Nairobi where modt of my friends live or work.