Text properties overriden in org-agenda header

Question

I do load a sexp entry - as a diary- from one of my agenda files which calls a custom function. Everything works fine except that the propertized text using utf glyphs is overriden in the agenda view as org-agenda-calendar-sexp.

The entry in the org file looks like:

* Data for a given day
     :PROPERTIES:
     :CATEGORY: data
     :END:

%%(myFunction)

Where myFunction returns a string made out of a data concatenation gathered from several weather forecast network places, which, simplified, looks like this:

(concat "test line :\t" 
        #("\uf055" 0 1 (font-lock-face (:family "Weather Icons" :height 1.2 ))))

Once the agenda is built, the line is there with correct data but:

What I expect is having the line with the glyph displayed using Weather Icon font family, as it does in any other buffer using font-lock if I insert that string.

What I get is the same line using my system default unicode font family for \uf055 glyph, as org-agenda-calendar-sexp face.

Question is

How I can keep the properties I've set first? Alternatively, where can I find %%() implementation?.

Related relevant questions which I've read before asking:

• Why does insert not insert colorized text?

• Propertize org-agenda-overriding-header

  accepted answer's section two may work, but I'd rather give up using propertized text before using such convoluted approach for a small thing like this.