that's not an exception. it's a actually a rule. try speaking hachisai quickly, it's then easily pronounced as hassai.
That's why there is this sign of 'tsu' in the place if you write it out in hiragana.
We all can guess and theorize, but it would be nice to have an actual Japanese speaker (maybe one of the developers), giving us a true educated answer, and lock the debate, and if needed to fix the exercise.