Some meeting members get "lost" when clicking on meeting link (different meeting room?)

When using Teams I create breakout rooms by making extra meeting links. The attendees are all from the same (external to me) organization and their organization creates the 'main' meeting.  During the meeting we split up in bo rooms. I created the links to the bo rooms. Most attendees can enter the bo rooms without trouble but some get 'lost'. They click on the bo room link I created and get the message that the should be invited to the meeting while their collegues can enter the bo rooms without trouble. They are not in the lobby, they are just kind of lost.

Can anyone explain to me why this is happening? This happens every time with this particular organization. Hope someone can help me out. 

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