Last Output does not show in command blocks in certain cases

The LastOutput line on command blocks sometimes does not show up. Some commands make the line disappear, some make it appear. When the output is turned off by switching O to X, the output line shows (however of course without any output).

Test:

1. Give command block
2. Place command block
3. Look in command block to see the LastOutput line is not there
4. Input

/testfor @p

and run to see now the line appears
5. Input

/say @p

and run to see now the line is gone