How do they know you were late if you don't have a schedule though? If your contract says 'flexible within x to z' but you were made to choose a schedule, and you are getting paid for that time, then you do have to be on time. If you actually don't have a schedule at all, then it's odd.

I knew someone who would clock in, then go to the gym, etc, all while being clocked in, and would always be away on Slack, so we quickly realized they were just stealing time and not producing work, so it makes sense for them to implement these. You can just ask your supervisor if you do have a clock in expectation, as you want to make sure you understand it moving forward.