@fool Thanks for the query parameter idea. It could work to help identify the request, indeed. Although, I understand that the request is prepared and sent by Netlify systems. I’m not sure to see how we could hook into it to add a query parameter.
Anyway, by checking further with our infra provider, we finally found that they were actually blocking one IP address which is among the ones used by Netlify build system.
Unblocking it appears to have solved the issue we were experiencing.
So, in the end, nothing wrong with Netlify.
Thank you very much for your availability, patience and help to solve that issue.