I buy directly from Rodents on the Road and never have an issue.
Almost all rats will wee a little when they die. Most of the rats also wee when the snakes constrict them. So I wouldn't think that a rat urine scene would bother a snake used to eating rodents.
That said, could it be that the rats also thawed some and then re-froze? Maybe the rat itself smelled off too?
When I've killed larger quantities of rodents at one time, I make sure they have bedding so any urine tends to go into the bedding and doesn't pool up to get all over the rodents. But they do sometimes pile on top of one another(as even live rodents tend to do) and the urine will soak the bottom ones.
Maybe try a good rinse first? A little blow drying, some brushing to fluff the hair, maybe do a little styling... wait.. too much? Too much.
But yes, try rinsing it off good if you find another one like that. A good frozen thawed rodent shouldn't smell "off" especially to our poor noses.