I’ve had Fitbit as well as an LG smartwatch strap starting to split after a year or two of use, and I just heard that a similar type of cracking happens with the synthetic PVC leather of gaming chairs, and the advice is to apply a good UV filtering spray like you get for dashboards of cars. A typical brand mentioned is 303 UV Protectant Spray which lubricates the material and filters UV. You’d probably want to apply this at least monthly but it may go a long way to making the material last a lot longer. There are similar sprays produced for tents and other outdoor furniture. But its too late to try this after the damage is done, it needs to be applied from just after to acquire the devices.
It is widely recognized that ultraviolet rays (UVR) in sunlight (wavelengths between 280 nm and 400 nm) is an important factor causing photodegradation to some organic substances such as polymers. UV radiation causes photooxidative degradation which results in breaking of the polymer chains, produces free radical and reduces the molecular weight, causing deterioration of mechanical properties and leading to useless materials, after an unpredictable time.
See https://www.sciencedirect.com/science/article/pii/S1658365514000880
#^https://www.sciencedirect.com/science/article/pii/S1658365514000880