groupby does not capture all values

[klahnakoski]

Example that works:

{
    "from":"task.task.tags",
    "limit":1000,
    "select":{
        "aggregate":"union",
        "name":"group_id",
        "value":"task.group.id"
    },
    "where":[
        {"eq":{"name":"description"}},
        {"find":{"task.tags.value":"Mozilla-LDAP|"}},
        {"gte":{"action.start_time":{"date":"1-jul-2019"}}},
        {"eq":{"treeherder.symbol":"rt"}}
    ]
}

replacing select with a groupby clause will show less values. Why?