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?