Since the issue is caused by an undefined behavior optimization, possibly the tests ran on a different optimization level or on a different architecture target? Would still be surprising, but less so than there not being tests at all.

Maybe pushing updates to users is their test process? I doubt they're the only ones doing this, sadly.

There is a policy that would probably catch these types of errors - Chromeos developers should eat their own dog food, i.e., developers should be required to use a Chromebook on the "dev" or "beta" channels as their daily machines. Sure, use a cloud VM to get work done, but login, email, browsing, etc. done on the Chromebook. Google itself has fine machines for this purpose.