You ignore it and get it to recalculate once around the unpaved road.
Tried that, went 10km away, still would not either calculate a new route or re-calculate the existing one - had to send me through that field. It would send me down bridal ways even when normal roads where available, which was also annoying.
Do you have recalc set to Auto, Prompted or Off?
Stopping and reloading the course can also fix dodgy routing once you've traveled past the glitch.
@Dammit started
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You ignore it and get it to recalculate once around the unpaved road.