Sorry, my bad. Figured it out. Cheers.

expected [ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'F_', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ] // This is your wrong answer
to deeply equal [ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'FM', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ] // This is the right answer

That's a problem with your code. Print the input and debug it. Or add this test to sample tests and see your code fail:

assert.deepEqual(solution("abba"), ["ab", "ba"]);

There is no problem with the tests:

JavaScript Completions 36104

When I press "Test", it returns "You have passed all of the tests! :)".

However, when I press "Attempt", it returns an error in Random tests.

Take a close look at the expected results of any random test:

expected

[ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'F_', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ]

to deeply equal

[ 'Mn', 'Wc', 'kU', 'Xp', 'yw', 'fx', 'Uf', 'qp', 'FM', 'FH', 'vX', 'DU', 'Py', 'HI', 'DT', 'jO' ]

If you say there's no problem in random tests, how come 'F_' is supposed to equal 'FM'?!

No, there is no problem in random tests in javascript. That's your code's problem, read this: https://docs.codewars.com/training/troubleshooting/

If your regex doesn't match anything, it returns null, and you then try to use .map on it (so you get this error).

There seems to be a mistake in random tests when you hit "Attempt"