Function Builder not working when only one value?

I have a functionBuilder

@_functionBuilder
struct MyBuilder {
    static func buildBlock(_ numbers: Int...) -> Int {
        var result = 0
        for number in numbers {
            result += number * 2
        }
        return result
    }
}

Function

func myFunc(@MyBuilder builder: () -> Int) -> Int {
    builder()
}

Use

let a = myFunc {
    10
    20
}
print(a) // print 60 is work!

but

let b = myFunc {
    10
}
print(b) // print 10?

Why is b not 20?

I try add other buildBlock

static func buildBlock(number: Int) -> Int {
    return number * 2
}

But not working 😟

Any idea?