Re: text properties not applied (almost a bug report, but...)

[rgb]

>    I doesn't depend on the mode, but on whether font lock is used or not
>    (font lock uses the face property).
>
> I'm not sure this is correct, since if you save my message,
> kill the buffer, reopen it, and run `font-lock-fontify-buffer'
> then faces change, but the code snippet still changes
> the faces when it is run.

When font lock is turned on, application of a text property
is seen as a change that causes the text to be re-fontified.
So your setting of the text face is almost immediately erased.
If you execute your code in a buffer with font-lock turned off
your face setting will persist.