A regular member of the group (ie, not a group manager) will see any upcoming activity under 2 conditions:
1. The signup period is still open, or
2. You are signed up for the activity
If you are signed up, you will see that activity through the date of that activity.
If you are not signed up, you will see that activity only until the signup ends. This makes sense, since the signup period has ended (you cannot sign up anymore) and you did not sign up while the signup was open. There’s no reason to show that activity anymore.