%session.Data killed after redirect

Any ideas on why my %session.Data array would be wiped out after setting %response.Redirect in OnPreHTTP()?

Context: trying to implement a SSO patter for authorization code flow.  I mention this because it's all I can really pinpoint as to what's changing.  Typically a user logs in by providing their username/password and we validate in our database and set the %session.Data property with the user info.  The only difference now is that we start at the SSO sign on page and are redirected back to our auth page where I'm calling our authentication server with the auth code provided to get an ID token.  For now, I've simply hard coded the %session.Data to the user I want.  

I have tested this and set a global right before the redirect to check the %session.Data and it's as expected (i.e. there's a user, IP adddress, and all the things we need to check that a user is 'logged in'.  The redirect parameter is set as the last thing before the quit in the OnPreHttp() method and we are redirected to a page where the first the OnPreHttp() method does is checks the %session.Data.  In my logic, if it's empty it redirects back to the authentication page (ours not the auth server).  

I have made sure I'm not running into any kills for the %session variable nor can I see any of the reasons a %session might be terminated: https://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY...

My guesses are:

1) The session is being terminated somewhere in the background that I can see or debug

2) The auth server's response is setting or clearing cookies, thus losing the session ID being stored on the browser

3) Magic and gremlins 

Thanks for any thoughts you may have!