Hi.
We faced the same issues in customer webinars over and over again. Not only green blocks, but also multicolour flickering artifacts, both on our and the customer's side. So we had to immediately change the platform. Pretty embarrassing for us annoying 300+ participants once more.
Fun fact: we encountered the issues not only recently but also 1-2 years ago and thought that we had connection or bandwidth problems, but that was not case. So we switched to a Streaming platform and everything was fine.
The internal IT suggested giving the new tool Teams Townhall another try, but we're facing the same results again.
It's really a pity, because the idea of the tool itself with it's functionalities is really good, especially when working in the MS365 cosmos anyways.