This is less about technicalities (the threshold is already excluded, so it doesn't affect other players' scores or graphs), and more about the concept that solutions work in the general case. If you need to change your algorithm whenever new test cases are added, then I don't consider it a valid solution, and I'll work to improve the test cases until it is invalidated.
Brute-forcing is actually the easiest one for me - it's evident from the graphs and doesn't require a "prove or disprove" process (recall that I handle players' solutions as black boxes). And D8 is not special in this respect, previous cases that I've invalidated included a solution that assumed out-degree <= 5 or so (I'm sure that it was out of the box...)
Note that this approach doesn't preclude loop unrolling as an optimization strategy. The game does favor minimal and elegant solutions (that's why cost is calculated as it is, and why certain achievements exist), but score is determined equally as a function of cost and cycles (i.e. commutatively).