I think I may have found a gem here:

EN (machine translation, emphasis mine):

Article 3/5.[¹ The communication of federal administrative authorities is clear and recognizable.
Federal administrative bodies communicate in a politically and commercially neutral manner.
The obligation to communicate in a politically neutral manner shall not apply to the administrative bodies referred to in the second paragraph of Article 1(f). ]¹

What about that exception? We have:

1°[¹ administrative instance:
…
(f) the federal government's strategic bodies referred to in the Royal Decree of 19 July 2001 on the installation of the federal public services strategic bodies and relating to the personnel of the federal public services designated to form part of the cabinet of a member of a government or a college of a Community or Region; ]¹

I’m not going on a chase to dig that up. But I would like to know if this means all “SPF …” agencies (SPF Economy, SPF Mobility, SPF Finances, SPF Foreign Afairs, etc) are exempt from commercial neutrality.

I also wonder if this apparently accidental legal effect is also accidentally nullified by this clause:

Art. 9. Where the application for advertising relates to an administrative document of an administrative [¹ instance]¹ [² ...]² including a work protected by copyright, the authorization of the author or of the person to whom the rights of the author were transferred is not required to authorize the on-site consultation of the document or to provide explanations about it.

Because I suspect that shitty corps like Facebook have a clause that transfers copyright to Facebook, in which case a request to liberate FB publications by a public service can be brushed off. But then that raises another question. In Belgium, copyright holders cannot transfer their copyright (which is actually to protect the human creator). E.g. the creator of the Smurfs cartoon retains copyright ownership. But then if my understanding is true, does that mean Belgian law is catoring just for the corner case of copyright being transferred outside of Belgium?

Do you have links to the source?

It should be in here:

https://refli.be/fr/lex/1994000357

AFAIK there is no English translation published.