Contract with Aleksei doesn't affect your JSS negatively. Please, note that there are multiple factors that contribute to your Job Success score and you can find a lot of information about how it's calculated here.
We won't be able to share specific details about which contracts contribute negatively to your JSS to protect privacy of clients' feedback.
You said that it does not affect the JSS of the freelancer, but It looks strange for me - his JSS droped in one day just after I canceled the contract with him. Doesn't seem like a coincidence...
Freelacers' JSS is updated on Sundays every two weeks. Kenny completed other contracts beside yours in the two-week period before the most recent score update.
Moreover, every time it updates both recent activity and activity over longer period of time can cause changes in the score. For example since JSS is calculated over 24-, 12- and 6-month windows, a score can change because a contract fell of of the calculation. Some contracts don't show on freelancers' public profiles because less than $1 was paid on them or a full refund was issued but they will still contribute to the Job Success score. Additionally, long-term relationships, contracts without feedback and inactive contracts can contribute to the calculation.
I hope it helps carify the matter.