Arguments highlighted as jsArrowFunction inconsistently

[kevinoid]

Requisite minimal reproducible example, formatted as plain text :

const a = () => null;
const b = ( ) => null;
const c = _ => null;
const d = (_) => null;
const e = (x) => null;
const f = (x, y) => null;

With Vim 8.1 (from vim-gtk3 2:8.1.2269-1 on Debian) empty arguments without whitespace, and a single argument named _ without parens are highlighted as jsArrowFunction while empty arguments with whitespace, a single argument named _ with parens, and one or more arguments of any other name are not, as displayed in the following screenshot:

Thanks for considering,
Kevin