(Replying to my own mail since I haven't received Stefan's answer, maybe
because I'm not subscribed to the list)

> > - M-: (add-text-properties 1 3 '(face '(italic :weight bold))) RET
> This is wrong.  It should be
> 
>     (add-text-properties 1 3 '(face '(italic (:weight bold))))
> 
> I.e. a face property ca hold either a "face" or a list of "face"s,
> where "face" can be either a symbol or a plist of face attributes.
> So you can set the `face` property to
> 
>     italic
>     
>     (:weight bold)
> 
>     (italic (:weight bold))
>     
>     ((:weight bold) italic)
> 
>     (italic)
>     
>     ...
>     
> but if the value you used above did what you intended, it's just
> a lucky accident.

Yes, I eventually did figure out that this only works incidentally.  The
only reason I tested this specific invokation is because the problematic
text in Org buffers had

    (:strike-through t org-level-1)

So I replaced the call to font-lock-prepend-text-property with
font-lock-append-text-property as a silly workaround.

I since took a look at font-lock-prepend-text-property and cooked up the
attached patch; if it is correct, I guess font-lock-append-text-property
could use something similar.