LinkedIn Software Engineer interview questions

The whole process is tough, two code algorithms never seen in Leetcode or somewhere else. At last there was a System Design, all the projects I write on resume is about backend, however, during the design interview, the interviewer kept asking me questions about all kinds of frontend UI problem, I told him, I dont have much frontend experience, but he ignored it and kept asking questions. There was another engineer also participate the interview, but she was just a shadow, even she said may be we can skip this frontend part and ask some system scale related problem, but my interviewer still insisted on finishing all his fronend question...At last, left me around 10mins to talk about system scalability, availability, consistency, database, etc, but seems he was not interested at all!! I totally doubt why I apply for bankend position, and ask a frontend engineer to interview me.