Goseur7 I don't think anyone really knows. In the context of GrapheneOS, people usually use secondary user profiles as a means to do compartmentalization into security domains. In that case, I do argue the shared loopback interface is a security flaw that should get fixed. However, Google never intended secondary user profiles to be used for compartmentalization, but instead to allow different physical users to share a single phone. In that context, the shared loopback interface would likely not be seen as a problem at all.
I think the GrapheneOS developers are planning to create separate loopback interfaces per profile, or at least isolate the communication in some sense: https://github.com/GrapheneOS/os-issue-tracker/issues/4772
So, in the context of GrapheneOS, this app might stop working in the future. But it most likely will keep working for a few more years, since this is not a prioritized issue. And we can hope that actual proper built-in support for copying or moving files between user profiles is implemented before then.