has16m is false but terminal supports Truecolor #62

I found this issue using chalk, but it's color support related so I thought it was a better idea to submit my issue here.

Check out the following code. The same string is colored using the default chalk instance, a chalk forced to level 2, and a chalk forced to level 3.

// Default chalk
let chalk = require('chalk');
console.log(chalk.supportsColor);
print(chalk);

// 256 colors chalk
let ch256 = new chalk.constructor({level: 2});
print(ch256);

// Truecolor chalk
let ch16m = new chalk.constructor({level: 3});
print(ch16m);

function print(c) {
    let result = '';
    for(let i =0; i < 60; i++){
        result += c.rgb(255 - 2*i, 0, 2*i)('█');
    }
    console.log('    ' + result);
}

The supportsColor returns this: { level: 2, hasBasic: true, has256: true, has16m: false }, so it looks like my terminal does not support Truecolor.

However, the printed colors look like that:

It is clear that my terminal actually supports Truecolor. Why is it not detected ?

What if I always force level 3 in my code ? Can it impact users who only support 256 colors ?

Thanks a lot

supports-color version: 2.0.0 chalk version: 2.0.1 Ubuntu 16.04.2 LTS gnome-terminal 3.18.3