If I set Website Restrictions to Limit Adult Content, Safari stops showing search engine suggestions regardless of the engine being used (Bing, Yahoo, Google, or DuckDuckGo). If I change the setting back to All Websites, it starts working again. Search suggestions do show up if I make the query from the search engine's webpage.
I'm currently seeing this behavior on an iPhone 5S running iOS 11.2.6, but I've also seen it on other devices and OSs. If this is a known issue to Apple, I have yet to find a public statement about it.
How do I make search suggestions work when iOS restrictions are enabled?