Your example should absolutely return false, but I doubt that the random test cases are wrong. They have been checked a lot of times already.

There is a problem with your code, though. And there isn't a specific test case to catch that particular bug. (Wish I could add test cases after the kata was published.) With a bit of (bad?) luck, the random tests won't expose it either.

Try running your code on isMerge('ananas', 'anas', 'na'). It should return true.

Thanks for your feedback! :-)