Solder Mask on fPCB starting to degrade
Photos of your degraded flex-PCBs would be helpful in diagnosing the issue.

You need to figure out if your PCB degredation is due to electical or environmental stress.

Environmental stress could be things like high ambient temperature, humidity, UV, corrosive environments (high salt content) and the like. It could also be application-related stress - excessive mechanical stress or flexure. 

Electrical stress is less likely - usually solder mask degredation is caused by heat (excessive current) or breakdown due to excessive voltage causing arc-over. Your description of a low-voltage sensor doesn't fit this bill.

If you can't establish a sure environmental or electrical cause, it would be worth your while to get in touch with the flex PCB vendor and ask for some failure analysis. They should be able to establish the cause of the degredation.