To be honest, I am surprised that I am even writing this, but after the amount of time it took me to resolve the issue, I thought more on the Internet should have easy access to the solution.

About a month ago, I was getting told that someone in senior leadership was still getting an email from our Advanced Approval rules even though they had already been deactivated in Salesforce. After checking that they were, in fact, deactivated, I looked into the Approval Rule to see what was going on. The user was not directly the approver since the rule in question had a Group as the Approver. I removed the Approver record, but that didn’t result in any change. I checked the group, and initially, I didn’t see the user because they were deactivated.

I went back to the user record and saw that the group was still listed on the user detail page. So I went back to the Group detail and selected to see ALL members and then saw the person. Once I removed the user from the group, they no longer received any emails.

Example:

This is a very odd bug, as I cannot imagine that this is by design, but TLDR is that when you deactivate a user if they were part of a group and that group is part of an Advanced Approval rule, they WILL get emails until they are completely removed from the group.