I don't understand why people defend the way it "works".
For the vast majority of players a horse is all about the five speed related traits. With your 90+ AH take two pure 5 speed, no extra trait, horses, breed them and you'll get extra traits. Excluding corrupted there are 13 other traits which have the possibility of being introduced. Of those 13 traits, 8 are negative traits. That's 8/13 or 61.5% chance that the first newly introduced trait will be bad and 3 of those traits directly affect speed, that's a 23% chance that the first introduced trait will affect the speed negatively. Then of course there is the chance that RNG may lose you one of those speed traits anyway. Don't even try it with 8 trait horses that don't fully match, I gave up with that when breeding the new horse colours way back. Regularly getting 3 and 4 speed horses only contributed to my butchering skill.
Now try breeding that pair of 5 speed horses with only 50 AH, you only have RNG to worry about as no extra trait is going to be introduced. You are regularly going to produce a foal that is better than that produced by the poor sap with 90+ AH.
Bug or bad game mechanic? Having worked in software development for 30 years before becoming a publican I can't think of a single customer who would have been happy with such a "mechanic". As for my customers now! Let me see, I'll put your beer into a larger glass and there's a chance you'll get an extra trait to make it tastier, but there's a 61.5% chance that something's going to taste like crap as well.