Principal came 50 minutes late to my formal observation/evaluation…

I had a formal observation yesterday that was planned at a 11:00am to watch my work station rotations for my special education class. My work station rotations are from 11:00-11:30. I spent so long planning and prepping a lesson I wanted them to watch me teach and give me feedback on. Nope! They missed my rotation block and came in 10 minutes before lunch at 11:50 while we were having a 15 minute song break. I told my admin that we were scheduled for 11:00 and that my academic block is over, they still went ahead and did my formal observation during us watching songs…how is this fair?