Safari 11.1 Search Engine Preference Reverts to Google

I found a solution in this Apple Discussion thread.

It turns out that there are two places to select the search engine. Setting the search engine from Safari's URL bar overrides whatever's set in Safari > Preferences > Search. My Safari had Google set from the URL bar, and whatever I set in Safari Preferences was immediately reverted to Google.

To reset the default search engine in Safari's URL bar, click on the magnifying glass icon, which appears at the left side of the URL bar when no page is displayed. A dropdown appears. Then select which engine you'd choose. Luckily, I was content with DDG, one of the only four choices presented.