What if safari thinks it's cross-tracking?

Question

safari

I have an application that hosts both the client side and the server side on heroku. They have different domain names, but they're both hosted on Heroku, so why does Safari still think it's cross-tracking?
[client].herokuapp.com [server].herokuapp.com
Important cookies are not stored because of this.
Please tell me what to do? Maybe you can somehow place both the server and the client on Heroku on the same domain name?