Here we go again! Just over a month since the second bearing was fitted and that one has now gone.
I've been researching yet further on this and a key point is a recommendation on GM-volt - make sure the dealer has the latest updates and copies of "Bulletin PIP5081".
Also, they are now recommended to perform a full flush of the gearbox after fitting, not just a refill.
There is also an indication that a first failed bearing should be followed with a replacement housing cover, and a second by a complete replacement box, but I am not sure if those are just things that have happened in some cases or if it is part of the protocol.
If I were you, I would cross examine the service manager on these points like a barrister in the High Court. Get him to show you the bulletin and the tools and how he will do the job.