Visual code debugger adapter immediately dies when used with vscode-hack debugger extension

I am using the vscode-hack extension on visual code studio on macOS Big Sur. My settings.json includes following launch configuration:

"configurations": [
{
"name": "HHVM: Run Script",
"type": "hhvm",
"request": "launch",
"script": "${file}",
"hhvmPath": "/usr/local/bin/hhvm",
"hhvmArgs": ["--mode", "vsdebug", "--vsDebugPort", 8999],
"cwd":"${workspaceFolder}"
}
]

When I try to debug a Hack script in Visual code debugger, Visual code debugger adapter launches and immediately dies. I have verified launching hhvm with the same args from command line, and it works fine. I am following the instructions here https://github.com/slackhq/vscode-hack/blob/master/docs/debugging.md.

vscode-hack extension version:
2.10.0

Visual Code version:
Version: 1.60.0
Commit: e7d7e9a9348e6a8cc8c03f877d39cb72e5dfb1ff
OS: Darwin x64 20.6.0

Os version:
macOS Big Sur 11.5.2

Reproduce the failure using the launch configuration in the issue and the vscode-hack debugging instructions. Compare the extension's debugger-adapter launch with the working command-line HHVM invocation; done means the adapter stays running and the VS Code debugger attaches to the Hack script.