I was working on some leetcode problems to practice go, bu I encountered a weird issue when I range over a map. I am not sure if it's a bug or a skill issue on my part (probably the 2nd)

type TreeNode struct{
  Val int
  Left *TreeNode
  Right *TreeNode
}
func levelOrder(root *TreeNode) [][]int {
    retMap := rec(root, 0)
    var retList [][]int
    for _, val := range retMap{
        retList = append(retList, val)
    }
    return retList
}
func rec(root *TreeNode, level int) map[int][]int{
    if root == nil{
        return nil
    }
    retMap := make(map[int][]int)
    retMap[level] = []int{root.Val}

    leftMap := rec(root.Left, level+1)
    rightMap := rec(root.Right, level+1)
    
    for lvl := range leftMap{
        retMap[lvl] = append(retMap[lvl], leftMap[lvl]...)
    }
    for lvl := range rightMap{
        retMap[lvl] = append(retMap[lvl], rightMap[lvl]...)
    }
    return retMap
}

In the function `levelOrder` the order of val is random.
If the tree is:

0 --------1-------
1 ------2 - 3-----

Sometimes I get `[ [1], [2, 3] ]` and sometimes i get `[ [2, 3], [1] ]`

If i use

for i:=0; i<len(retMap); i++

as the bounds of the for loop, the values are return in the correct order.
Is this a skill issue on my part? Or is the range operator not supposed to be used like that?

Screen Shot of this in my terminal