Symptoms
When participants join your Zoom meeting that is configured for pre-assigned breakout rooms, some users are not put into their assigned rooms.
Resolution
Meeting participants should open the Zoom app and sign in before joining your meeting, or sign in after joining the meeting and then enter the breakout room themselves.
Cause
For pre-assigned breakout rooms, only users who are logged in to the accounts that you pre-assigned will be put into the rooms you set up. By default, when you join a Zoom meeting via a link, you will join the meeting as a guest, unless you have logged into the Zoom app first.