In rec/spec, the full chat window hotkey is repurposed as the All Visible hotkey, as you noticed. You can still click the button at the top right of the screen to display the full chat window. I wonder if people use the diplomacy window or the full chat window more often in rec/spec... maybe they can be swapped later. Sorry for the trouble!
It's ok, I just like to see what people said in the lobby before the game started when watching recs, so I press alt-t at the start of the game instinctively