because of a partnership.

This is the only possible explanation.

After changing the setting, close Opera fully and open it again to see if Google sticks.

Changes occur after updating the browser, extensions and the system have nothing to do with it. I have not noticed similar behavior in other browsers.

Opera sometimes sets Yandex as the default search engine in certain countries (like Russia, Kazakhstan, and a few others) because of a partnership. It can also happen if your browser locale is set to one of those regions, or if an extension/malware is changing it.

Try these steps in order:

• Check extensions
  Type opera://extensions in the address bar. Disable or remove anything you don’t recognize (especially search-related ones).

• Set Google as default again
  Go to opera://settings/searchEngines.
  Make sure Google is set as the default under

• “Search engine used in the address bar.”
  Also check the list and remove any unwanted engines if possible.

• Hide the start page search bar (if Yandex shows there)

• Go to opera://settings/startPage and turn off the search field on the start page. Then always search from the address bar (top of the browser).
  Check your language/locale

• Go to opera://settings/languages. Put English (or your preferred language) at the top and set it as the display language. Restart Opera.
  Extra checks if it still changes

• Scan your computer with Windows Defender or another antivirus for unwanted software.

• Try creating a new Opera profile (or reinstall Opera completely) to test if the problem stays.

After changing the setting, close Opera fully and open it again to see if Google sticks.