Athletics Champs competitions are usually set up with "Manage automatically per team" startgroups. This means that an identical startgroup is automatically created for each team. For this type of competition, it is customary for all events to be run in a Combined-events format. As a result, this Startgroup setting does not work in combination with Single events.
How to solve this: change the Startgroups to "Manually manage". However, this easily leads to mistakes, so make sure that if you change an athlete from a team, you also change the athlete from the startgroup.
This has been automatically translated into English