Indeed, I can reproduce it.
It depends on the width of the drop down => see the resizer edge on the completion drop down? Make it wider (does just have to fit the beginning of the arguments, not all of them). And then when you select the line, it will preview the args correctly.
Same in 4.0RC2
Not yet sure why...
Please report it as a bug (so it wont be forgotten). You can refer to this forum post.
Thanks.