EmberJS JSONAPIAdapter with hasMany + embedded relationships

In addition to my last question ember.js JSONAPIAdapter with hasMany a colleague asked if the "kind-of"-sideloaded relationships in the working JSON:API structure could be embedded like this:

{
    "data": [
      {
        "type": "altersgruppe",
        "id": "1",
        "attributes": {
          "name": "UNTER_21"
        },
        "relationships": {
          "tarifbeitraege": {
            "data": [
              {
                "type": "tarifbeitrag",
                "id": "3",
                "attributes": {
                  "name": "ZAHN70",
                  "beitrag": "3-29,70",
                  "proergaenzung": "7,00",
                  "gesamtbeitrag": "25.99"
                }
              },
              {
                "type": "tarifbeitrag",
                "id": "4",
                "attributes": {
                  "name": "ZAHN90",
                  "beitrag": "4-28,70",
                  "proergaenzung": "7,00",
                  "gesamtbeitrag": "30.99"
                }
              }
            ]
          }
        }
      },
      {
        "type": "altersgruppe",
        "id": "2",
        "attributes": {
          "name": "ALTER_21_24"
        },
        "relationships":{
          "tarifbeitraege": {
            "data": [
              {
                "type": "tarifbeitrag",
                "id": "1",
                "attributes": {
                  "name": "ZAHN70",
                  "beitrag": "1-25,70",
                  "proergaenzung": "7,00",
                  "gesamtbeitrag": "25.99"
                }
              },
              {
                "type": "tarifbeitrag",
                "id": "2",
                "attributes": {
                  "name": "ZAHN90",
                  "beitrag": "2-25,70",
                  "proergaenzung": "7,00",
                  "gesamtbeitrag": "25.99"
                }
              }]
          }
        }
      }
    ]
}

The idea behind that: We can use relationships with less problems in the java backend (where sideloaded structures are harder to implement).

But the JSON above structure does not work. The store contains only the first level of data, which is "altersgruppe", but "tarifbeitraege" are empty.