Right. With that in mind the solution is smaller, and passes all test cases. Thanks for your patience!

Your task is to return the FIRST longest string consisting of k CONSECUTIVE strings taken in the array.

You're missing the consecutive part. No faulty test.
'ixoyx', '3452' and 'zzzzzzzzzzzz' are one after the other, 'wkppv', 'ixoyx', and 'zzzzzzzzzzzz' are not ('3452' is between the last 2).

OK, even when carefully keeping the order, there's a faulty test case:

['it', 'wkppv', 'ixoyx', '3452', 'zzzzzzzzzzzz'] 3
'wkppvixoyxzzzzzzzzzzzz' should equal 'ixoyx3452zzzzzzzzzzzz'

The 3 longest strings are (in order): 'wkppv', 'ixoyx', and 'zzzzzzzzzzzz',
so the expected result should not be 'ixoyx3452zzzzzzzzzzzz'.

No, what he did was sorting the strings in descending length order, then returning 2 joined, what is wrong. But what you said maybe help him to clarify what to do in that case.

I experienced this too. You must have encountered something like this [boat, shoes, boat, shoe], where the expected is boatshoes yet your code returned shoesboat. I think it must return the first consecutive longest string, in this case boatshoes.

You're changing the order of the strings. The expected value is ok.

Some test cases are simply wrong. Eg.

'owiaxujylentrklctozmymuwlwsasphmxx' should equal 'wlwsasphmxxowiaxujylentrklctozmymu'

which should not.