That is true, please look at the example in the kata's description. For the case you say it can't be true, the explanation would be:

'acab' is a merge from 'aa' and 'cb':

    s:  a c a b   = acab
part1:  a   a     = aa
part2:    c   b   = cb

There is nothing to fix, you can't reorder the letters, read the description again.