Changing my solution to add an additional string did pass the random attempt tests. Presumably then the Codewars interface was flipping the specific string passed in as input to use as expected output and undoing the flipping done by my previous solution to leave it the same, whereas the sample tests have separate input and output strings, confusing at the time but I think that all makes sense now. Thanks for your responses.

Language is C++

Expected result strings don't seem to be getting words flipped on Random_Sentences_Tests when making attempts. For example for the random input "still like of for pizza they are I are" my script produces "llits like of for azzip they are I are" but the expected string is "still like of for pizza they are I are".