After enquiring on the payload returned from Ory Network, I found that I should have created my community relationships this way:

{
  "namespace": "UserList",
  "object": "copains",
  "relation": "community",
  "subject_set": {
    "namespace": "Community",
    "object": "com",
    "relation": ""
  }
}

instead of this way:

{
  "namespace": "UserList",
  "object": "copains",
  "relation": "community",
  "subject_id": "com"
}

After using the proper way my issue is fixed! Maybe this way of doing should be more documented as using an empty relation was not intuitive to me.

Thanks for sharing this. I'm running into something similar with traversal and subjectsets. I'll see if I can apply this to my usecase.