best response confirmed by ThereseSolimeno (Microsoft)
Hello again, why have you added them as guests if you only want to have meetings and not any requirement to access Teams resources etc? You could either remove the guest accounts because then you're already set with the open federation or you can ask the guests not being able to access the meeting to make sure they are signed into your tenant from the org. switcher accessible from the top right corner. That's my best resolution to this. If still not working you better open up a support case.