It looks like the API is broken.
Specifically, it looks like making changes to the account owner user using a full access API token is broken.
I have confirmed this by importing the account owner via Terraform and making edits to the user. It fails with “PUT API call to https://api.pagerduty.com/users/ failed 403 Forbidden. Code: 2010, Errors: , Message: Access Denied”
There is also this issue in Github, confirming that others are seeing the same thing: