Additional features, settings, or issues not covered by specific Microsoft Teams categories
I have just had the same problem with the disappearing icon. I have used breakout rooms perfectly successfully in several remote lessons. The set up for today's lesson was exactly the same - I was the meeting organiser; the class were all in the meeting as participants - but the icon simply wasn't there. I looked under the three dots and in settings, but couldn't find it anywhere.
I sent the class off to call each other in their pairs instead. Then I left and rejoined the meeting, and the icon was back. Really random. Not ideal when you're working with a big class of 14-year-olds!