The dealership told that's normal for this model.

 

Here's a answer from AI................
AI Overview

The 2023 GLE 450's backup warning system, including Parktronic and Blind Spot Assist with Exit Warning, may not always provide audible alerts when vehicles approach from the side or rear while backing up. Specifically, the audible alerts for Parktronic are typically limited to close proximity to stationary objects during parking maneuvers. Blind Spot Assist primarily uses visual warnings in the side mirrors and may not include audible alerts for vehicles approaching from the side when backing up.