Safari 11.1 running under macOS Sierra 10.12.6., on a mid-2013 MacBook Air. The software is all up to date.
I'd like Safari to use DuckDuckGo as its default search engine. However, when I choose DDG in the dropdown at Safari > Preferences > Search, the dropdown immediately and spontaneously reverts to Google, without further input from me. All other Safari functionality works correctly. Interestingly, I have exactly the same software running on a mid-2010 MacBook Pro, and that Safari allows search engine selection.
I have only two extensions (wipr and Omnikey) installed; removing them did not affect this behavior. Quitting Safari and the OS had no effect. Restarting in Safe Mode had no effect. Trashing the com.apple.safari.plist (from ~/Library/Preferences/) had no effect. Replacing the .plist with the plist from the correctly-operating MBP had no effect.
Any ideas how to remedy this?