Depending on the specific Jagwire cable set, both the cable exterior and housing interior are made of slippery substances, so you shouldn't be getting any grinding. In fact, even with older cables, if installed properly, lack of lube may make the brakes harder to apply, but shouldn't grind... Back in the day, I didn't always lubricate before assembly, and never got grinding.

I could envision a bad cut of the housing that could cause the metal used to strengthen the housing to scrape the cable... but you would have probably noticed this pretty quickly. You should explore any "interfaces" with cable to housing, barrel adjusters or cable guides to see where the problem is happening... Especially the front brake should be pretty straight forward to inspect, since there are only 2 points for a potential issue.