This is a very good question, and one often discussed in the forums.
First, to be fair to Aeotec, the latest generation of the multi sensor in the instruction manual says to disable the motion sensor if you want to use it outdoors. It can be used outdoors as a light sensor or temperature sensor but you will get too many false alerts if you try to use motion. So the manufacturer not only knows it, they tell customers that it’s not intended for outdoor use as a motion sensor.
The reason applies to all PIR motion sensors. These devices don’t actually measure motion, they measure very small changes in heat as it moves across the sensor. For this reason, they are very easily triggered even by a cloud moving across the sun.
Depending on the specific use case, there are other technologies that can be used or you can set up a zone with multiple motion sensors although outdoors you can still get false alarms on multiples from a warm breeze.