Hi Rudy,

You nailed it!! Using property EXPORT_LATEX_HEADER exactly solves my 
problem, thank you very much. For completeness, I will re-attach the 
file with the solution

How did you know that, should that be mentioned in the corresponding 
section? (I could not find another one) [1] [2]. Maybe that two links 
together should be more linked and more explained the subtree case.

Serve at least this discussion as a pre-documentation

Regarding the attached file: I thought it was minimal enough, 94 lines 
that runs on vanilla emacs (emacs-30 branch); but maybe I missed a 
little summary: it is two subtree exports handled with a ~#+begin_src 
emacs-lisp~, and there was a clash using the LATEX_HEADER

Cheers,
Pedro

[1] https://orgmode.org/manual/LaTeX-header-and-sectioning.html

[2] https://orgmode.org/manual/Export-Settings.html