I’ve flagged this to the team! The app is meant to show a single Live Activity that lists all your running agents, so seeing multiple cards at once is not intended.
In the meantime, swiping away the extra cards on the lock screen will clear them, and they should not come back until you start a new agent.
One question that would help us narrow this down: does it tend to show up after an agent has been running for a long time (several hours), or right after you open the app and immediately send a message? Either detail would be really useful.