goTenna Full Stack Software Engineer interview questions

Had a first round chat with the Lead Talent Acquisition at GoTenna. The call lasted for about 30 minutes and consisted of basic and standard questions. After I was given a full stack assignment to complete within a week. The assignment was reasonable and included several bonus points of which I chose to implement the authentication point. In response to the completed assignment they wrote: ' ...the team was able to review your technical assessment and the results were very strong and the team is hoping to facilitate additional conversations. " So we scheduled another interview round call with a Full Stack Engineer: '. .. It will definitely be more technical in nature. They will ask some general algorithm questions, ask you more about your tech stack and why you prefer using different tools. They will want to learn more about your experience with databases, front end, and back end server side development as well as learning more about your goals and interests. ' The conversation went well and consisted of several technical questions, nothing too complicated. I provided answers to all the questions and interviewer seemed to be impressed. What bothers me most and why I chose to provide a negative feedback, is because the interviewer eventually decided to turn me down based on my bonus point implementation: "The lack of understanding encryption, and no authentication API experience really were areas the team was hoping for more exposure.." How can you turn down a candidate based on a bonus point? Why have me spend a whole week to complete an assignment, even complete a bonus point and then turn me down based on that bonus point?