Let's say I have an org file with this content (this is the Org Manual):

    *** System-wide header arguments
    #+vindex: org-babel-default-header-args

The file is opened normally, highlighted and everything.

Now, as a test I try to add a display property to the S character of System:

    (put-text-property (point) (1+ (point)) 'display "aaa")

It works. The S is replaced with aaa.

But if I try the same on a character on the vindex line then nothing happens.

Why is that?