In your example you knew the issues with the original fix, had some ideas to the cause, even if they were wrong, and generally knew where to look.

In my experience the LLM when given the ticket would have done the original null pointer guard fix given the bug. Only under direction does it ever dig deeper and for me it'll often go down some wrong paths unless I tell it to go somewhere else. It's great when it gets it right the first time. But that is rarely the case and usually you just get good enough if you don't care to go further.