[SalesForce] Approval Process Assignment Email Alert Not Sending

I'm having difficulty figuring out why this approval process isn't sending an email to the approver. Here are the details on the process:

• Active: Yes
• Next Automated Approver Determined By: (this is empty)
• Entry Criteria: (this is empty)
• Approval Assignment Email Template: My_Approval_Assignment_Template
• Initial Submission Actions:
  • Record Lock
• Approval Steps
  • First – Criteria: Custom_Field__c EQUALS 'Required', else Approve, Assigned Approver: Assign to Approver__c
• Final Approval Actions
  • Record Lock
  • Email Alert: Notify Submitter
  • Field Update: Mark Approved
• Final Rejection Actions
  • Record Lock
  • Email Alert: Notify Submitter
  • Field Update: Mark Rejected

The problem is My_Approval_Assignemnt_Template doesn't get sent out to Approver__c in approval step 1. Here is what I have tried:

1. I have verified that the email address is valid.
2. I have verified that the user is actually assigned the approval process.
3. I have verified that the approval/rejection emails go out properly.
4. I have verified that email deliverability is 'all email.'
5. I have switched the submitter and approver users and the approval/rejection emails still behave as expected, but the assignment alert is still not sent.

Am I missing something? I saw here that they mention the 'next automated approver' should be filled out, but for my process I don't need that, I just need a specific calculated approver in the Approver__c lookup field on my object. Are those two fields tied together?