I'd guess that it's because they (incorrectly) think it's an embedded Webview, which get blocked (see https://developers.googleblog.com/2021/06/upcoming-security-... and https://developers.google.com/identity/protocols/oauth2/poli...).

You could try creating an issue in the Cloud Identity issue tracker (Cloud Identity is Google's API for letting websites have a "Login with Google" thing): https://issuetracker.google.com/issues/new?component=522910&...