Coming from the Land Rover world (I know, I know), my concern would be what would cause the issue other than a design defect, such as improper oiling, improper bearing seating design, or improper materials? In other words, is it possible that the failures have isolated causes or is it more endemic of a defect that runs across the entire design?
And it it is one of the former, what can fix it other than opening up the engine and replacing key components or even a redesign of the short block?
Land Rover has a Ford-built turbo diesel V6 that was barely sold in the U.S. but is a big part of the foreign markets. It's known for crank failures, and the smartest analysis points to the oiling being insufficient. The crank is shorter than ideal because the business-arm of Ford and JLR wanted the engine used in so many different vehicle models that it had to be a shorter block, and thus overruled the engineering-arm. The shorter crank led to compromises being made in oiling - compromises that the engineers probably thought were barely sufficient but it turns out were actually insufficient.
I know that bringing up Land Rover in a discussion on Toyota reliability is like bringing up Sherwin Williams in a discussion about Matisse, but it's the only other case I know of crank-related failures so soon in an engine's life cycle. In fact, the Land Rover ones generally last until over 60k miles.
The way I see it there's only a few ways that bearings fail in this way:
1 The bearings are not matched - ie the two parts that rotate against each other are not sufficiently matched and cylindrical so they hit each each other or the fluid film provided by the oil cannot keep them separated.
2 The oil flow is insufficient to maintain the fluid film thickness to keep the parts from direct contact. - Or 2.1 - Oil flow is too high. This can cause over-cooling > increase friction > thermal runaway > bearing failure.
3. The oil lacks the necessary properties maintain the fluid film thickness. This could be the wrong oil choice, the wrong additive package, air bubbles in the oil from an oil pump cavitation problem or some other issue in the oil system introducing air bubbles to the oil supply. Could also be moisture collecting in the oil for some reason or other contaminant like over-fuel conditions or low friction piston rings resulting in gasoline passing into the oil.
4. Cooling is insufficient and the heat causes expansion of one or both bearing components causing the clearance to expand or shrink causing either a wobble or a seizing.
5. The rotating assembly is not balanced or aligned with the outer bearing surface causing contact. This could be either an issue with the crank forging, heat treating, or machining or an problem with the engine block warping under heat, being cast incorrectly, being bored incorrectly or out of alignment, or being assembled incorrectly.
6. Incorrect materials - bearing surfaces being too soft is probably most common, but possibly also too hard(?)
7. Incorrect machining. If a bearing surface is machined too smooth it may not have sufficient microscopic valleys or porosity as engineered to retain oil in the material surface below the plateaus of the machined finish. Polishing too smooth can result in short lifespan.
8. Contamination in the lubricant or surfaces. This is pretty obvious one - anything larger than the fluid film thickness will cause problems.
All of these failure modes can be pretty quickly summarized into a few categories: The parts don't fit, the oil is contaminated, the oil isn't getting where it needs to be, the parts are getting too hot.
Contaminated oil is the one that's easiest to fix without an engine manufacturing change. Parts that don't fit is also usually a pretty easy fix - machine the parts better. Parts getting too hot is usually a symptom of the other stuff. And the really hard ones to fix are oil that isn't going where it needs to - that may require a partial or full engine oiling redesign. If this becomes a persistent long-term problem, I'd put my money on the problem being oil not going where it needs to go in enough volume/pressure. If it's any of the others, I'd expect it to be fixed pretty quickly.
Just my 0.02 as someone who's never seen the inside of one of these engines and just speculating from the pictures and a background in mechanical systems. An automotive machining expert or triboligist could probably add a lot to help better speculate on the issue. I'm sure Toyota has a team full of superstars in both fields.
