Mosaic interview questions

Phone call interview to start. Questions we're fairly boiler-plate, but it was a good conversation and the HR manager invited me for an on-site interview. I sent a thank you note and my availability for the specific day she wanted me to come in, and she responded saying they would be in touch to confirm an exact time. However, less than an hour later I received an auto-reject email saying I was being passed on. I found this perplexing as she made no indication that was the case and I'm not sure why she would ask me to come in for an on-site interview if she wasn't interested in moving me forward to the next round. I tried following up asking for simple feedback and never heard back. I found very unprofessional. It's not hard to respond back to an email or a quick phone call.

The initial interview is with the CTO. Followed by 2 technical interviews. 1st was to create an API and write some test cases for the endpoints. 2nd was about data structures. Both were live and not too difficult.