Similar (but not precisely the same) issue here with 3 different magnetic cables. My wife and i each have a ZU GPe and they are in pristine condition kept in flip cover cases. We have been using the magnetic charging cables for months. However, the past couple of weeks the cables no longer charge at the normal rate.
After being attached, using the Sony transformer, for hours, either phone only shows a charge increase of about 10%. Had the USB connector been used, the battery would have been fully charged. I have made no changes to either phone on stock GPe ROM. Also, I have checked for any corrosion on the connectors and found none.
For now, we use the Sony USB cable and I'll test again after we get Android L.