I ran into an issue today with our MTR that has the new UI enabled -- when attempting to "Cast" to the MTR from Teams desktop or iOS apps, the Chat pane never finishes loading but instead shows a message like "We couldn't load your messages. Try accessing it again later." (Not great grammar, but that's the least of the issue.)
This happens regardless of whether choosing "Just Cast" or "Cast and Join", and regardless of whether casting screen share or PowerPoint.
Casting from desktop Teams app to at least one of our MTRs that does not have the new UI enabled (but that is the same updated Teams version on the MTR) does work. However, Casting to this MTR from iOS Teams app does not work. Sigh.
My GCC org previously had this issue for like 3 months prior to the new UI release, but it had been working as of a few weeks after the new UI release. It sucks to see that it is again broken in some cases. (And MS Support, who I had a ticket open with for like 3 months, never officially acknowledged it as a bug, nor did they tell me when the fix was rolled out. Double sigh.)