Code folding fail for arrow function when param list has no parentheses

[golopot]

TypeScript Version: ???

VS Code version: Code - OSS 1.38.0 (3db7e09, 2019-09-05T13:09:16.557Z)

OS version: Linux x64 5.2.8-arch1-1-ARCH

Search Terms:
code fold
folding
arrow function
parenthesis

Code

// expected
// curly block is foldable
testThing(
  (a) => {

  }
)

// unexpected
// curly block is **not** foldable 
testThing(
  a => {

  }
)

Expected behavior:
vscode let me fold the arrow function body.

Actual behavior:
It does not.

Playground Link:
Not applicable.

Related Issues:
no