More like absorbed moisture in the DOT 3 or DOT4 brake fluid.I'm wondering if salt, winter slush, and lack of use because of regen has created a corrosion or pitting of the piston surface.
Could also be a dud seal = rare.
Reason why I flush the brake fluid every 2 years or 40,000Km per old Australian / Japanese car maintenance schedule and guess what..., re lube slides & even with high end of life km's and keeping vehicles 15+ I've never had to replace any brake hydraulic components ever.
Other people who have skimped on brake fluid flushes (Or their workshop never ever actually performed them) have needed a bank loan for a HUGE brake job besides the usual pads and discs - which on a BEV EV we should never wear out
What I do see is some vehicles much worse than others - maybe the venting of the brake fluid reservoir ?
Think the Volts (2013) does a better job than most - when changed at 2 years...... (remember there is also the ABS which is also difficult to flush without a dealer computer and thorough technician).
GM schedule 5 years .... too late as it can be a cloud corrosion soup from moister ingress over time by then.