I've been in the business for ~20 years and there are very few cases I can recall where it really mattered what the underlying sorting algorithm was, and no cases where the actual details of the algorithm's implementation mattered. Not saying these cases never come up--I am saying they are exceedingly rare [EDIT: and often company-specific], and making them general candidate interview questions might be a bit of a waste of time.

Present a candidate with a simple system with three levels of middleware API and have them plumb a newly exposed low-level value up through all three levels to the high-level API. This is unfortunately 75% of professional software development.