You are unable to populate breakout groups using Course Sets from Blackboard Learn.
After clicking the dropdown to select Course Group Set, you are told that authenticating with Blackboard was successful, but there is no option to choose the group from the course in Blackboard Learn.
Blackboard are aware and working on a fix. There’s currently no ETA on when this will be available.
More detailed information is available on Blackboard’s knowledgebase.