I have reported the chat issue on a separate thread here: Group ride chat funkiness---can only see some of the messages
Feel free to vote on it if you are also encountering the issue. I have also reported the issue directly to Zwift support. If you have a few minutes and feel strongly about it, please do it too. 
The communication aspect with other riders in a group ride, sometimes very large ones, has long been one of the most attractive features of Zwift to me. Now sending and receiving messages is a bit like playing the Russian roulette. 