Hi @aardvarq and @alejo_menocal,
The warning here is indicating that the module does not declare the provider name in required_providers, so the solution is to ensure that all modules have the providers they expect to use declared via required_providers. This allows terraform to statically validate the usage of providers in the configuration, preventing many common errors and confusing results.
While we left this as a warning for compatibility reasons, the documentation strongly suggests that all modules declare the specific providers they require. See: Provider Requirements and Providers Within Modules