Exactly same behavior happening with me as of Feb 2024.
I explicitly unchecked the option to not make Opera my default browser. I'd really like to have it for occasional use. But Opera keeps becoming the default browser somehow.
It happened a week ago and I uninstalled and re-installed. For a few days after installing, Chrome was the default browser and everything worked fine. I just realized that my web links suddenly started opening in Opera. Default browser in Windows settings has changed somehow. I did not click anything in Opera or in Windows to make it the default. Spooky!
So uninstalling it for the third time and may be for good now.