Defining "auto_complete_set_by" attribute to GitPullRequest object doesn't make PR auto-completed

Hi, I'm seeing an unexpected behaviour when I try to create a PR that auto-completes.
This is a snippet:

import azure.devops as azdo 
import azure.devops.released.git as azdo_git
...

pr = azdo_git.GitPullRequest(
                    auto_complete_set_by=(
                    azdo_git.IdentityRef(id="<some id>")
                    ),
                    title='title',
                    description='description',
                    source_ref_name='source_ref_name',
                    target_ref_name='target_ref_name',
                    completion_options=azdo_git.GitPullRequestCompletionOptions(
                        # also tried with merge_strategy=False,  same result
                        merge_strategy="noFastForward", 
                        delete_source_branch=True, triggered_by_auto_complete=True
                    ),
                    reviewers=self.set_pr_reviewers(self.pr_reviewer_ids),
                )

azdo_connection = azdo.connection.Connection(base_url=self.organization_url, creds=credentials)
git_client = azdo_connection.clients.get_git_client()

repository_id = "<some repo id>"
project_id = "<some project id>"

git_client.create_pull_request(
                pr,
                repository_id,
                project_id,
            )

I have made sure that the user ID for auto-complete is correct (the one passed into IdentityRef())

the resulting PR is successfully created, all looks good except it's not auto-completed. The PR has no attached pipeline or branch policy, so I can't figure out why it won't work.

Any help is appreciated, thanks!

Start with the GitPullRequest.auto_complete_set_by and GitPullRequestCompletionOptions fields, then trace how git_client.create_pull_request sends them. Compare the resulting request with Azure DevOps auto-complete requirements; done means a created pull request auto-completes under the stated conditions. No file or test is identified in the issue.