is array.sort() implementation incosistent?

I would like to give some feedback about ...

From: https://js.checkio.org/mission/most-wanted-letter/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.117 Safari/537.36

My code:

function mostWanted(data) {
    return (!data) ? '' : data.toLowerCase().replace(/[^\w]/g, '')
                                  .split('').sort().join('')
                                  .match(/(.)\1*/g).sort((a, b) => b.length - a.length)//[0][0] Uncomment this to get the most wanted letter
}

The issue :

The code should preserve the order after the second sort, it does that untill it reaches 11 unique characters.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

"If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behavior, thus, not all browsers"

I understand that, but it's odd that changes after a set ammount of unique characters.

I would know: Is this intended?

// First time I encounter the issue
>>>mostWanted("Lorem ipsum dolor sit amet")
<<<["ooo","mmm","ee","ii","ll","rr","ss","tt","a","d","p","u"]

// Expected result
// ["mmm", "ooo", "ee", "ii", "ll", "rr", "ss", "tt", "a", "d", "p", "u"]

I checked this in the chrome browser and it works.

console.log("Lorem ipsum dolor sit amet".toLowerCase().replace(/[^\w]/g, '')
                                          .split('').sort().join('')
                                          .match(/(.)\1*/g).sort((a, b) => b.length - a.length))
(12) ["mmm", "ooo", "ee", "ii", "ll", "rr", "ss", "tt", "a", "d", "p", "u"]

Examples:

// This works fine...
>>>mostWanted("OOOMMM")
<<<["mmm","ooo"]

// This works fine...
>>>mostWanted("aOOObMMMc")
<<<["mmm","ooo","a","b","c"]

// This works fine...
>>>mostWanted("abOOOcdMMMef")
<<<["mmm","ooo","a","b","c","d","e","f"]

// this doesn't work anymore, 
>>>mostWanted("abcOOOefgMMMhil")
<<<["ooo","mmm","c","e","f","b","h","i","l","a","g"]

// I think after 10 unique character changes the sorting method. This doens't work
>>>mostWanted("abcdefghiOOOMMM")
<<<["ooo","mmm","c","d","e","b","g","h","i","a","f"]

// With 10 works
>>>mostWanted("abcdefghOOOMMM")
<<<["mmm","ooo","a","b","c","d","e","f","g","h"]

// With 10 unique character but 11 character works just fine
>>>mostWanted("AAbcdefghOOOMMM")
<<<["mmm","ooo","aa","b","c","d","e","f","g","h"]