Regarding "best practices, improving, or conducting code reviews": Here "improving or conducting code reviews" is supposed to refer to questions like "How do I properly conduct code reviews", not "Please review/improve this piece of code". The latter is of course perfectly on topic and what this site is for.
The distinction between code correctness and debugging is supposed to be that in the former case you post code that you think is correct (because it compiles and gave you the correct result for your test input) and you're asking us whether we see any problems with the code (like a logic flaw which leads to wrong output for certain edge cases - or maybe a memory leak or something like that). Note that useful response to that question might be something along the lines of "There will be wrong output in this one case" without actually telling you how to fix it (obviously people will tell you how to fix it if they know, but it's perfectly valid for people to just point out the mistake if they don't immediately know how to fix it).
If you want to fix a specific bug this is different because you're asking people to find one specific bug for you instead of just generally looking for problems. The basic workflow of this site is supposed to be that people look through your code and give you their 2 cents on any possible problems they see (be it possible bugs, readability problems or possible performance bottlenecks). If you restrict the question to one specific bug, that runs counter to that workflow. Also the question being specific makes it on-topic on Stack Overflow.