Indeed... I did some insanely stupid brute force logging/debugging and figured out that I had a problem with distinguishing nil and empty slices. Really sorry for making a fuss.

Sorry, there is no typo.
Please see my answer to @Tuffk above.

Your are printing some given arrays but not all so the answer you got is for arrays that you didn't print hence didn't see.
When you post code or tests put the "Spoiler" tag (I marked it for you).