I know it's a different car, but I've seen a very similar example with a prius. The terminals and bus bars develop a small amount of corrosion over time, which can easily be cleaned off once disassembled, in the same way that you can clean corrosion off a normal 12V battery. Because there are a lot of contact points and a lot of current, the voltage drop (and power dissipated) can be enough to cause fault codes with what appears to be very little visually. With the corrosion removed, the example I saw was back to full health, but in the case of a faulty cell it would also be possilbe to replace that (rather than the whole assembly).
Here's the album of the work, showing step by step what can go on inside the battery, how to test cells and how to fix corrosion induced problems:
Lexus Gs450H Hybrid(Traction) Battery
in Lexus GS 300 / Lexus GS 250 / Lexus GS 430 / Lexus 450h / Lexus GS 460 Club
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This is amazing stuff! Thank you so much.