If a session with pre-assigned breakout rooms is created and started, any newly added breakout rooms with assigned users won't reflect immediately during the session due to how Samba Live's system works.
Every time a session is started, the pre-assigned breakout rooms are cached. Caches are kept for 5 minutes, meaning new breakout rooms won't be added until the cache is cleared. The breakout rooms will keep showing as per the cache, and it won't include the newly edited pre-assigned rooms.
Waiting 5 minutes to add new edited pre-assigned breakout rooms is a good option.
However, if waiting is not possible due to time restrictions, the issue can be avoided by ending the session altogether. This action will clear the cache immediately, letting you add more pre-assigned breakout rooms and users without any problem.
Note: Closing the browser tab won't be enough to clear the cache. Using the End Session feature is required.