Not sure I agree with your assessment of Toyota's build quality being "poor" (and no I don't own a Toyota), but 50K is too soon. Even whatever car line you personally think is the best have parts failures.
Was it the seal that failed and the bearing replaced as a precaution, or did the bearing get noisy (and you always replace a seal that you molest in the course of other work)? If the seal, they can get nicked on assembly. Bearings are vender sourced and made by the millions but surprisingly, few fail prematurely. If the bearing, was the bearing really toasted and failed in a major way, or just noisy?
I suppose the bottom line to your questions is, "Could this have been lubricant related?" IMO, doubtful. Mostly likely it's just a "lucky you" moment when the universe takes a nip out of yer bum for fun.
Other possibilities? In my experience, input shaft bearings fail earliest on cars that idle a lot. The input shaft and bearing is turning when the engine runs but, generally speaking, does get much lubrication until the tranny is fully in operation, car going down the road. Essentially, it relies on the lube it gets going down the road to carry it during idling periods. If the car idles for inordinate periods, the bearing can fail sooner than it might normally. There's very little load on it at idle, so even if idling a more than "normal," it's not a terribly likely cause. I do not know the exact internal layout of the Corolla tranny and am speaking generally.