例如当用户SomeUser向电子邮件some_user@gmail.com和任何密码注册时，可能是这种情况.然后，他使用同一电子邮件的Google帐户登录，然后尝试使用其Google帐户的电子邮件再次登录.在这种情况下，在用户将其Firebase帐户链接到google帐户后，google会删除密码.此处一些详细信息

This may be when for example user SomeUser registered with email some_user@gmail.com and any password. Then he signed in with google account of the same email and then tries to log in again using email of his google account. In this case google removes password after user links his firebase account to google account. Here are some details