The default autocomplete profile has been changed to include sub phrase matching. Essentially this means the autocomplete matching can start at various points within the page title, rather than only at the very start. This has shown to be particularly useful on sites with long or nested page titles like mw.org and wikitech.

This is only the default, if you don't enjoy the new behaviour it can be changed back to the default profile (or other, stricter profiles) from the Special:Preferences Search tab.

Related ticket: https://phabricator.wikimedia.org/T212788

Erik B.