Cannot set invisible text property

[Enrico Flor]

In a minor mode I wrote for myself, I use add-text-properties to make
certain things invisible.  This does not work in org-mode, and I don't
understand why.  For instance, the following has no effect in when the
buffer is in org-mode (and only when it's in org mode):

    (add-text-properties 1 3 '(invisible t))

Any other text property I try to add does not give me any problem.
I have also tried to achieve the same by adding things to
buffer-invisibility-spec, but again, the following has no effect
whatsoever, if in org-mode:

    (add-text-properties 1 3 '(invisible hide-this-stuff))
    (add-to-invisibility-spec '(hide-this-stuff))

Can anyone give me a hint as to what is going on?

Thank you.

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