System User not created under Client Business when using On Behalf Of Flow

I am implementing the On Behalf Of flow as described here.

After Step 2's call to /<CLIENT_BM_ID>/access_token?scope=<scopes>&app_id=<APP_ID>&access_token=<PARTNER_BM_ADMIN_SYSTEM_USER_ACCESS_TOKEN>, I expect my app's system user Id to show up under the client's System Users (under Business Settings).

However, this never happens even though Step 2 returned the access_token as specified by the document.

How do I create a system user under the client's business manager?

When I call /me/accounts with the access token of the system user I created, it returns a page which I have access to (the one I connected in Step 4. However, making calls using the page_token retrieved result in the error:

"Cannot call API for on behalf of user :

The app I'm using is a test app.

My main question is how do I give my system user access to the client's assets?