If you try to sign in to your account in the Kagi app for Android with Opera as your default browser, Opera opens as expected with the page to select your Google account, but then, the redirect to the Kagi app doesn't occur. As a result, you can't sign in. You have to temporarily switch to Chrome as default to sign in successfully.
Opera usually works fine when it comes to redirects and signing into apps. Across the apps I'm using, Kagi is the only one where it doesn't work.