Maybe you could post your code (mark it as "Spoiler").

Note that Code Wars is for those Learning to Program all the way up to Experts. It should be considered bad practice to assume that anyone attempting a kata, especially an "easier" one, would know all programming best practices. Instructions are there for a reason and should be explicit.

It is known that in any code it is bad practice to modify the input. It is not particular to this kata. If you want to mutate, mutate a copy. Note that lots of guys passed the kata (see at the top of the page).

That's probably something that should be in the instructions

If you modify the input you will fail all the random tests even if you pass the fixed ones. I just tried a few solutions and had no problem.

all 11 of the kata tests pass, it's only the random ones that fail, all 200

I think you modify the input.

All 200 Random failures look like :

Expected: '[]', instead got: '[['VUxATV', 'VsKkvL VGR osu LSVY chE'], ['VUxATV VsKkvL', 'VGR osu LSVY chE'], ['VUxATV VsKkvL VGR', 'osu LSVY chE'], ['VUxATV VsKkvL VGR osu', 'LSVY chE'], ['VUxATV VsKkvL VGR osu LSVY', 'chE']]'

but all Samples are green

All the random tests are failing, sample tests work fine but the random tests are expecting and empty array