As former email admin who has put up with this, you are breaking cardinal rule about mixing marketing and transactional emails. Marketing should be different IP/Domain from transaction emails.
This BS is pedaled by the email tools who want you to have less friction setting up DNS when onboarding to their tool (less conflicts on a new subdomain).
If you just check the latest version of Google Postmaster Tools, you can see sends from your root domain and subdomain cross-polluting your reputation in Gmail. They aren't dumb.