Issue Type: Bug

I have two snippets (for latex documents) defined in latex.json snippets file :

{
    "pth": {
		"prefix": "pth",
		"body": "\\pth{$1}$0"
	},
    "interval": {
		"prefix": "interval",
		"body": "\\itv{${1:left}}{${2:right}}{${3:left_value}}{${4:right_value}}$0"
	}
}

There is something I don't understand concerning placeholders order.

If I use the "interval" snippet alone, there is no problem : using TAB, I can switch placeholders in expected order : 1:left->2:right->3:left_value->4:right_value->0.

But if I use the "pth"snippet, then, inside of this snippet, if I trigger the "interval" snippet, I would expect being able to browse placeholders in the same order than in the previous case, however it's not the case : the actual order is 1:left->2:right**->0->**3:left_value->4:right_value->0(from pth snippet)

Is this a bug or is there something I did not understand ?

VS Code version: Code 1.27.1 (5944e81, 2018-09-06T09:21:18.328Z)
OS version: Windows_NT x64 10.0.17134