According to Waze support, when using Carplay split-screen (where Waze splits screen with other app like music), Waze won't issue traffic Alerts by design!! That's true, I have confirmed it. As soon as Waze returns to "full screen", it resumes issuing traffic alerts (danger, weather, radar, etc).
Split-screen is a quite usefull feature of Car-Play as it simplifies greatly audio/music control on the road, without the issue of messing with several buttons.
Although in split-screen mode the screen area that Waze has to show information is reduced, there is no reason why Waze could not keep issuing the alerts by sound "danger ahead"" or by means of smaller icons.