Strips Guard is not designed for window/doors made of metal. In general, metal interferes with radio waves and can reduce the range for smart home devices. This means that if a metal object is placed between the unit and the controller, a metal door for instance your Strips sensor may not communicate with the gateway as it should, thus resulting in false open/close readings or not working altogether.
By placing the Strips sensor between metal surfaces, (e.g. between a metal door and a metal frame) you will pretty much block the signal completely and should be avoided when mounting your Strips sensor.
Another effect is that the Strips antenna gets very close to the metal surface due to the thin design which deteriorates the overall performance of the antenna. The magnet (or rather the magnet field) is also affected by metal and makes it almost impossible for the Strips sensor to detect.
We do however have a way to get around this! By just lifting the unit 3-5 mm with a spacer of wood or plastic improves the situation dramatically. If the sensor is lifted 3-5 mm from the metal surface it would help greatly. As rule of thumb, the distance between the magnet and Strips Guard should be shorter than between magnet and metal.
Note: It is imperative that the magnet and Strips Guard remain at the same level.
Here are a couple of pictures that show mounting the sensor on a metal door and frame with a plastic spacer.