In this particular case it was simply a matter of not enough corner cases defined. I was however using property-based testing, doing things like reversing then un-reversing the UTF-8 strings, re-ordering code points, merging strings, etc for verification. The datasets were in a variety of languages (including emojis) and so I mistakenly thought I had covered all the bases.

But thank you for the link, it's turning out to be a very enjoyable read! There already seems to be a few things I could do better thanks to the article, besides the fact that it codifies a lot of interesting approaches one can take to improve testing in general.

Nice! Glad to be of service.

Python, of all languages, probably has the best property based testing library out there with "hypothesis". I sometimes even use it to drive tests for my Haskell and OCaml and Rust code. The author of Hypothesis wrote a few nice articles about why his approach is better (and I agree), however I can't find them at the moment..