The “bug” is that the DNF margin is only supposed to appear if a horse actually has a “pulling up” injury (broken leg or heart attack). For some reason, sporadically, it’s deciding to throw that margin out at other points, when it’s supposed to actually show a number (distance) or – (if the horse is in last).
So yes, Andrea is right, unless your horse actually breaks a leg or dies (i.e. “does not finish” a race), ignore the DNF margin and substitute whatever margin makes you happy (the hair on his chinny chin chin, or 25 lengths, whatever).
Also, yes, it’s not like the code is flagging horse #592050 as “evil” and deciding to always DNF it. It is a random, sporadic bug, which is why it hasn’t been fixed because I haven’t been able to figure out what triggers it.