consider this situation

 | a | b | c | d |
 | .  | e |  . | f |
 |  . | g | h | i |

suppose i'm going from a to c, with a via point at e. the shortest would be to do a u-turn after visiting e, so following a,b,e,b,c

instead osrm goes all the way around the loop, following, a,b,e,g,h,i,f,d,e