In that extreme cross chain configuration you have the maximum amount of chain for the derailleur to take up. And you also have the least amount of chain tension. Combine that with on the least number of teeth engaged and you have the configuration that is most likely to jump.
Should it jump no.
It is a shop problem - maybe.
In just 11 months the spring in the derailleur could have weakened.
Or if you ride on the small gear in the rear even a bit it will wear and new chain might skip. But given it does not skip on other front gears I would chalk it up to chain tension.
The shop may have too many chain lengths. In biggest front and rear could a chain length be removed?
Cross chaining a road bike or the first two chain rings is something that happens. But on a mountain bike in the granny gear that extreme cross chaining is just not something that should come up. I would not put that kind of stress on a chain even to test.
Some front derailleur have a trim for big to big. This make sense to me as you may come to a stop but will get back up to speed soon. And now you have maximum teeth evolved. I don't know if trims comes in a 3 speed and it would make less sense as the outer is out further. Even with a trim cross chaining puts more stress on the chain and other components and is not as efficient.
The question is not about specifically about cross chaining but here is a link.
Technical Details and Implications of Cross Chaining
Not the question but the smaller rear gears are going to wear the fastest as they share the load across fewer teeth. So go for a larger front ring when you have choice. But still avoid cross chaining (a lot).