Both really. The problem is supposed to have been caused by misaligned robots at LG's factories:

https://arstechnica.com/cars/2021/08/misaligned-factory-robo...

But the fault of both GM and LG is that they weren't able to detect the problem sooner.

So better regression testing on new battery deliveries and better unit tests on the batteries themselves. That's a good item to add to the after action report. Now, what do you do to the battery to make the unit test? Shake it? Heat it? For how long?

I guess I'm just not seeing fault at GM here. Trying to get there though.

In this case you can x-ray it to see the problem. They were x-raying some, but not enough.

If you're going to outsource manufacture of key components -- like the battery for an electric car, which is the single most important and expensive component in the entire vehicle -- then the buck stops with you for quality control. People inside GM may want to blame LG, but everyone outside GM will blame GM.