i bought a beautiful blood bay with no markings, so i thought, hmm i'll add some markings with a reroll, and now she's a light bay! it said that only the markings and patterens would change. she has no markings whatsoever, but shes a light bay.

The only thing that can not change through a reroll is the actual genetics. The shade of the base color (in this case bay) can change. So when you rerolled your horse, the shade of the base color changed from blood to light.

This is not a bug. Rerolls have the ability to change the variants of coat color. For example, a blood bay can become a light bay or a pale palomino could become a chocolate palomino.

From the purchase page:
Unsatisfied with your horse's game picture? You can reroll their image and have their markings and patterns regenerated randomly. Only their markings and patterns (including things such as <font class="mainlineblack">shade of base coat and modifiers) will change, their genes will not change. Once purchased, you will see the option to reroll your horse's image on the horse's page.

I added the bold because bay, light bay and blood bay are all shades of bay. Chocolate and pale palomino the same way. Chestnut and liver chestnut. Buckskin and Buttermilk Buckskin. Just shades that are lighter or darker than "average". Like golden retrievers can be just about any shade from platinum to light brown. In the same litter. Real horse breeders can't usually breed a true line to produce a particular shade because the genes are the same, it's just variation between individuals. BTW, your horse can also change its status for sooty and pangere with a reroll and the rarity of its color can change.

It can also get birdcatcher spots, frosting, and chimera on refills.