identify_url config is ignored when using identity widget to authenticate

**Setup:**
We have a Hugo site hosted on Netlify here - https://www.stardog.com/
CMS works as expected.

We have added a subsite called Stardog Labs:
It's on Netlify here - https://stardog-labs.netlify.app/

On the main site `_redirects` file we are doing a reverse proxy
`/labs/* https://stardog-labs.netlify.app/:splat 200!`

This allows for our Labs subsite to render as expected here -
https://www.stardog.com/labs/

**Goal:**
We can continue to access the Labs CMS admin portal here https://stardog-labs.netlify.app/admin/

We would like to to access here - https://www.stardog.com/labs/admin/

**Problem:**
We keep receiving the following errors

**Additional setup info**
Here is the config.yml setup for the Labs site

**Closing questions:**
Is there something wrong with our config to allow this to work?
Are we missing a header like the 400 error indicates?