Challenges in Cross-Domain Tracking Between WordPress and React WebApp

I having difficulty establishing reliable cross domain tracking UTM and Referrer data between a Word Press site (which is hosted on my top level domain) and my React WebApp (hosted on a subdomain.) I can see anonymous users on the top level domain landing page with the UTM or referrer data but once the user goes to sign up and becomes named I lose both, I only begin to see UTM data if the known user comes back to the through the top level domain landing page. But that is to late for me as I'm trying to track account creations. I have in my React WebApp required config in my mixpanel init

 cross_subdomain_cookie: true,
 persistence: "cookie",
 cookie_domain: ".toplevel.domain"

I'm at a bit of a loss, I have tried multiple versions of a script on my WP site but I'm not seeing consistent results with persisting the cross domain tracking and having the users merged.