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

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'.

Some test cases are simply wrong. Eg.

'owiaxujylentrklctozmymuwlwsasphmxx' should equal 'wlwsasphmxxowiaxujylentrklctozmymu'

which should not.