When you click on to an activity that has comments, they do not appear unless you then refresh the page.
If you navigate to the activities page using the back button, or go back to it after visiting a different page, any planned activities disappear. It only shows completed activities. Again refreshing the page makes them appear again.
Can you always reproduce this? I tried myself now and it worked fine - that doesn’t mean there isn’t something wrong! The Intervals.icu app maintains a persistent connection to the server and chats (ride comments), activities, planned workouts etc are pushed from the server to the client. The connection can be broken and should re-connect. However there is plenty of scope for bugs there that would cause the behaviour you have seen.
After clicking a coached athlete’s activity, I still need to refresh the page to be able to load the comments, and after doing this, often the planned activities then disappear from the calendar.