Running multiple Visual Studio Tasks in Parallel

I'm trying to run a long-lived background task in visual studio (tsc --watch) while also running my app as two separate tasks. So, (app)-[dependsOn]->(tsc --watch). But, it appears that the app task waits for the tsc command to finish before it executes, which is a problem since it runs forever.

I'm simplifying my problem to show the actual issue I'm running into, but should hopefully guide me down the right path.

I have two task in my tasks.json file:

{
    "label": "Echo 1",
    "command": "echo",
    "type": "shell",
    "args": [
        "echo1"
    ],
    "group": {
        "isDefault": true,
        "kind": "build"
    },
    "dependsOrder": "parallel",
    "dependsOn": [
        "Sleep 5"
    ]
},
{
    "label": "Sleep 5",
    "type": "shell",
    "command": "sleep",
    "isBackground": true,
    "args": [
        "5"
    ]
}

Echo 1 -depends on-> Sleep 5

According to visual studio documentation the sleep task should be running in the background allowing the Echo task to do it's thing. But, whenever I execute the Echo 1 task, it waits until Sleep 5 is finished and THEN it actually executes the echo task.

> Executing task: sleep 5 <


Terminal will be reused by tasks, press any key to close it.

> Executing task: echo echo1 <

echo1

Terminal will be reused by tasks, press any key to close it.

I've tried adding the background property detailed in the documentation, but the editor shows a popup with the message Property background is not allowed. whenever I add that field.

Am I missing something else to ensure that the Sleep 5 task doesn't block the Echo 1 task?

I've tried already tried using the presentation with panel: dedicated/new to each task so they don't use the same terminal (not even sure if that's how visual studio executes it's tasks).