MathCo Product Engineer II - Backend interview questions

I applied to the Product Engineer II - Backend role on LinkedIn. I got a call from the hiring manager a week later who confirmed the information that I had provided while applying and scheduled a call 2 days later. I was told there would be 3 rounds in total (2 technical + 1 HR). In the first technical round - they asked me about my work experience. Later they gave me 2 coding questions 2 solve. 1 easy 1 medium. I was able to solve the easy one but I couldn't solve the medium one completely. Then comes the fun part, The interviewer doesn't even know what role he is hiring for. I had applied to Backend developer role but he keeps asking Frontend role. When I tell him this , he says they are hiring for Fullstack role , which I later tell him - they're not. I was told by the hiring manager to prepare for the Backend role and he specifically sent a mail with a set of tech stacks that I had to prepare for the role. I talked to the hiring manger about this and he agreed that it was indeed unfair and told me that he'd talk to the guy who asked all these questions. But I didn't hear back from him. In the end he rejected me (I got a rejection email even before the meeting had ended). This is probably what happens when an interviewer doesn't see an interview as something that'd change the candidate's life but rather a side task that he has to finish so that he can go do his work . I've interviewed to many companies before , and honestly , this is the worst experience I've ever had.