Maybe this illustrates the problem better:

func main() {
	loc, _ := time.LoadLocation("America/Santiago")

	t0 := time.Date(2020, 9, 5, 0, 0, 0, 0, loc)

	t1 := t0.AddDate(0, 0, 1)
	fmt.Printf("midnight=[%v]  day duration=%v\n", t1, t1.Sub(t0))

	t11 := t1.AddDate(0, 0, 1)
	fmt.Printf("midnight=[%v]  day duration=%v\n", t11, t11.Sub(t0))

	t2 := t0.AddDate(0, 0, 2)
	fmt.Printf("midnight=[%v]  day duration=%v\n", t2, t2.Sub(t0))
}

Adding a day twice does not produce the same result as adding two days.

The result:

midnight=[2020-09-05 23:00:00 -0400 -04]  day duration=23h0m0s
midnight=[2020-09-06 23:00:00 -0300 -03]  day duration=46h0m0s
midnight=[2020-09-07 00:00:00 -0300 -03]  day duration=47h0m0s

@ianlancetaylor

The result is surprising but this is working as documented. time.Time.AddDate will adjust the time, and then, as the documentation says, normalize the result just as is done by time.Date.

In your example we add one day to 2020-09-05 midnight. That gives us 2020-09-06 midnight. But in Chile there is no such time. There is 2020-09-06 23:59:59 -0400. And there is 2020-09-06 01:00:00 -0300. But there is no midnight. So we have to normalize. There are two plausible choices: 2020-09-05 23:00:00 -0400 and 2020-09-06 01:00:00 -0300. As the documentation for time.Date says, in this ambiguous cases we make no guarantee as to which one we select. In this case we happen to choose 2020-09-05 23:00:00, and the timezone for that has to be -0400. And that is the first 23 hour day.

Then the code adds 1 day to that. There result is unambiguous: adding one day to a time at 23:00:00, gives the next day at 23:00:00. In this case the range between those two times includes the skipped hour between midnight and 1am. So that is the second 23 hour day.

So although the results are confusing, there really is no bug. Unfortunately, at a daylight savings time transition, you need to figure out what you want to do. For example, instead of time.AddDate(0, 0, 1), you could write time.Add(24 * time.Hour). Then the difference would always be 24 hours, but your times would start being at 1am instead of midnight. You'll have to decide what is right for your situation.

Closing the issue because the code is working as documented.