I have got a strange issue in Magento 2.2 (I have checked in Magento 2.2.3zzz and Magento 2.2.4).
When a non-logged in user enters email in the checkout page, it checks the existence of email and if the record is found, Magento shows the password field for login.
The placeholder for the password field shows optional, but when the user tries to login without entering a password, the validation fails and it shows "
This is a required field." validation error.
Is this default problem in Magento or I have checked out a wrong repository?