I think it's only consistent in play.golang.org because results are cached: https://blog.golang.org/playground.

If I run it locally it's not consistent:

    go run lock/main.go
    [3 0 1 2 5 4 6 9 7 8] 10⏎
    
    go run lock/main.go
    [1 5 6 7 8 0] 6⏎
    
    go run lock/main.go
    [0 3 1 2 4 5 6 7 8] 9⏎

Here's a version that does work for me: https://play.golang.org/p/b6bRb9pgIGZ

The issue is that appending to slices concurrently is not safe, so you have to use a lock around the append or similar.