A Huawei executive has responded to a question from a user on Weibo about whether a particular case is appropriate for his newly purchased Huawei Mate 40. The case in question is partially made of metal, as can be seen in the image below, and that is where the problem lies. According to the executive, using a metal sleeve will seriously reduce the strength of the cellular signal as well as the GPS location since the metal will interfere with the sensors. To compensate for a weak signal, the phone will use more power to produce stronger signals, which in turn will cause more battery drain and shorter battery life.
Covers are generally made from mostly non-metallic materials such as plastic, fabric, or leather. However, it appears that some cases have metal to provide added strength. However, it is best to avoid these types of cases as they can adversely affect the performance of the phone.