Checkout.com interview questions

I had a first-round interview with talent acquisition, followed by a take-home test, and finally an in-person interview focused on system design. During all three steps, I received positive feedback. However, after a call from the HR team informing me that I had successfully passed all the steps, they suddenly told me I was not successful due to some gaps in system design and the coding challenge from the second-round interview. This felt quite strange; it seemed like they didn't know what they were doing. It is possible to successfully pass all interview stages and still not progress with your application, which I find unprofessional.