How common is it to be hired for a part time job described as a certain number of hours but then scheduled for considerably more?

I recently started a new job where I was told it was 20–24 hours per week, which is exactly what I was looking for as I have another obligation that takes up a lot of my time. Plus, I was told that the schedule is very flexible, I can pretty much choose my own hours and days. Sounds great. When I was asked for my availability, I said that my schedule is quite open but that I can only work up to 24 hours per week. My second real week on the job they had me work 6 days (once I was called in on my day off, not scheduled) and 3 of those days in a row were 10.5 hour shifts.

I messaged the person who does the schedule saying that I was concerned there may have been a miscommunication about my availability, very politely, and I said that while I can only work 24 hours, I can work virtually any shift or day. She did not return my message or ever get back to me in any way (this was almost a week ago now), though she did apparently passive aggressively say something to another employee about it.

Today I went online to check my schedule for next week and they have me scheduled for 6 days again, my only day off is one that I needed off for a medical appointment that was scheduled before I even got the job.

Is this normal? This seems insane to me. If they wanted to hire someone for full time hours like this or a schedule like this, why would they tell me they only wanted someone 20 hours a week? Am I being naive?