We don't know, since we don't have the code. Both are possible. Science knows I've written a shit-ton of incorrect code that accidentally worked until it sudden

We don't know, since we don't have the code. Both are possible. Science knows I've written a shit-ton of incorrect code that accidentally worked until it suddenly didn't. OTOH if you make an OS and you break API contracts that you had in the past, that would reasonably be expected to continue working, it's your fault. Blame is a judgement call.