I have found the solution in this thread: Whatsapp trigger not working, all tests are sucsessfull - #8 by Samueldev
One of the reasons that N8N’s trigger does not receive messages is that the phone number’s Whatsapp account is not subscribed to the Meta’s app. This sounds counter intuitive because everything seems to be correctly configured in Meta’s developer and business pages, but a simple test in Meta for Business (formerly Facebook for Business) can show if this is the case.
If a system token was created for the Whatsapp account, use it instead of the provided access token
A GET request to graph.facebook.com/<whatsapp account id> should return some information of the Whatsapp account
A GET request to graph.facebook.com/<whatsapp account id>/subscribed_apps should return the apps it is subscribed to. If an empty list is returned, it indicated that no apps are linked to this Whatsapp account and no messages will be received by the N8N’s Whatsapp trigger.
In order to solve this issue, a POST request should be send to graph.facebook.com/<whatsapp account id>/subscribed_apps. It should return "success": true if everything went ok.
When a new GET request to graph.facebook.com/<whatsapp account id>/subscribed_apps, it should return the data from the Meta’s app.
Hope this helps someone else. I spent a huge amount of time trying to debug the problem becase there is no indication that something is wrong in Meta’s dashboards.