eleugers,
When creating the breakout rooms during a meeting, you have three options:
1) Assign users automatically
2) Assign users manually
3) Let participants choose their own breakout room

For more information on this, check out our support article Managing Breakout Rooms
To avoid confusion, I would make sure you don't have any existing breakout room settings on that meeting by going to zoom.us and editing the settings under the "Meetings" tab.
If this helped you, please mark it as a solution so it may help others.
Brad