-- in.cue --
import "list"

issue563: {
    #MyDef: {
        name: string
        ...
    }

    _all: [
        _a,
        _b,
    ]

    _a: [...#MyDef] & [
        { name: "a" },
        { name: "b" },
        { name: "c" },
    ]

    _b: [...#MyDef] & [
        { name: "1" },
        { name: "2" },
        { name: "3" },
    ]

    output: [...#MyDef] & list.FlattenN(_all, 1)
}
-- out/list --
issue563: {
	#MyDef: {
		name: string
	}
	output: [{
		name: "a"
	}, {
		name: "b"
	}, {
		name: "c"
	}, {
		name: "1"
	}, {
		name: "2"
	}, {
		name: "3"
	}]
}