It timed out. Since this case is O(2^n) the fuzzer managed to build a relatively short input that caused the function to not terminate for a few minutes.
Oh wow, I misread your original post as O(n^2) and thought “no way a fuzz test would notice mere quadratic time complexity”, but exponential is another beast entirely :-D
Mere quadratic time complexity is something I would expect to timeout and catch at fuzzing time, as long as we do encourage non-tiny inputs, say 10K and beyond.
Edit: shortly after I wrote this comment, fuzzing discovered another pathological input - and that was fixed in https://github.com/elves/elvish/commit/04173ee8ab3c7fc4a9e79...
(In case people are curious, the project is a Unix shell, Elvish: https://elv.sh)