Identity verification for web fails for new users only

I set up identity verification for web about a year ago and it has been working fine until recently when I noticed that for new users the initial ping request fails with the following error:

{
  "type": "error.list",
  "request_id": "00008k...",
  "errors": [
    {
      "code": "invalid_document",
      "message": "either_email_or_user_id_present validation failed"
    }
  ]
}

My application sends email address only and generates a user_hash HMAC against the email only. I have verified that the new users are sending the correct email and user_hash HMAC using the hash checker tool in the admin web console. 

What’s odd is that older users pass validation and the ping request succeeds while newer users fail (same code path). I’ve compared the request/response for successful and failing users and I don’t see any difference. I’ve also tried a variety of new email addresses in case there is some email validation I’m unaware of but all new user emails consistently fail. Has anyone else experienced this?