Apparently it is a bug. It works for search engines shipped with Firefox (such as DuckSuckGo) but not for others (such as StartPage of Qwant). See https://twitter.com/gchampeau/status/938757076213518336
Here are some other HTTPS-enabled search engines to test. If I am not mistaken all have been mentioned on HN in the past.
https://wiby.me
https://gigablast.com
https://mojeek.com
https://unbubble.eu
https://ecosia.org
https://searx.me
https://symbolhound.com
https://findx.com