I,m not sure its a bug : Radar of the mig 21 is VERY WEAK, thus it is displayed furthur away (like a sa-10 would be displayed closer to you that a plane, even if they were at the same range).
As for the U, I don't know if the F15 RWR IRL is able to differenciate it from other planes, but its not needed : you know that if you have a UAV on RWR very far you should watch out for the fishbed, as there are very few other planes able to give this kind of warning, and even less that you'd meet in missions.