Please stop reporting this as a duplicate.
This issue is NOT the same as the issue "MC-36812 Chat Opacity Option is not working"

To recreate this bug simply open up a world and type "hello", or alternatively run a command block with a /say command.
You will see that the text will appear as it should, stay there for a few seconds and then suddenly disappear.
Before 1.7 the text didn't "suddenly disappear" but instead it faded out very gradually. I am fairly sure this is how it is supposed to be.