API v2: group_conversion_rate always 0.0

[grssnbchr]

I am querying the API v2 on a self-hosted instance with the following payload:

{
  "site_id": "xxx",
  "metrics": ["visitors", "conversion_rate", "group_conversion_rate"],
  "date_range": "day",
  "filters": [ ],
  "dimensions": [ "event:goal", "visit:device"],
  "order_by": [["conversion_rate", "desc"]]
}

I get this as response:

"results": [
        {
            "metrics": [
                39,
                9.68,
                0.0
            ],
            "dimensions": [
                "connection_search",
                "Mobile"
            ]
        },
        {
            "metrics": [
                38,
                9.43,
                0.0
            ],
            "dimensions": [
                "departure_place_search",
                "Mobile"
            ]
        },
...

As you can see, the group_conversion_rate is always 0.0. I don't understand.
The total visitor at this point in time is about 400, so 39/400 gives the conversion rate of about ~10%. However, out of these 400 visitors, about 230 are on Mobile, so the group_conversion_rate should be about 39/230 = 17%.

Am I doing something wrong / unintended with the API? I don't fully understand the instructions either.
When I submit an additional filter like "filters": [["is", "event:goal", ["departure_place_search"]]], the group conversion rate stays at 0%.