Hi François,
Thank you for getting in touch with us, we do appreciate your time.
Implementing shortcodes in a marker description is possible, as we do not remove any shortcodes from the marker description. You can try this by adding a map shortcode to your marker description.
With that said, it could be possible that the forms plugin that you are using is not rendering the shortcode, thus I would recommend reaching out to the developers of the forms plugin and ask if it is possible to add a shortcode in the way that you are doing so.
Note that marker infowindows are dynamic which means they are built/added to the HTML DOM when the respective function is triggered.
I hope this answers your question?
Kindest Regards,
Matthew