It seems the problem is about chat window size, when the message takes 4 or more lines in the chat field, the first line is dissaperiang, unless you resize the chat window before sending the message by this button (only a minor adjustment is required):
So, the workaround. If you chat window is something like this:
Please adjust it to something like this:
Or, in case you are writing a long message and still want a small chat window, you can resize a little the chat window before sending it and full message will be restored.