Unless contractually specified otherwise, it's generally up to the buyer to check the delivered goods for defects and report those without undue delay*. If this is not done, the goods are deemed to have been accepted.
Sure you can contractually specify that the product has to meet certain specs and pay extra for the seller performing QA, but the default often is "you're buying whatever comes out of our factory, check the goods yourself on delivery". The reason things are done this way in the business world is that it is generally cheaper to accept certain failure rates than to perform testing at every step of the supply chain and add a whole lot of bureaucracy and complications because of returns.
Whether custom contracts existed in this case is unknown, but it is likely that Infineon notifying customers was already a courtesy. They could've just said nothing.
* Under German law, which likely applies here since that's where Infineon sells from.