I was. I contacted them and asked. In the same way as you, I received a notification around noon, and my isolation period ended at midnight the same day. I had been in a restaurant on 13th Dec. At lunchtime on 23rd, I got the notification and then a couple of hours later a message saying my self-isolation ended by midnight. Like you I thought ‘odd’, but apparently, the Covid case I had contacted on 13th had only notified the app on 23rd that he/she had had a positive test.