One thing worth pointing out here is you that when reading you rarely will actually find the bug that you set out to, undoubtedly you'll notice others. Because you're reading the code with a mindset of "what awkward conditions failed to be handled appropriately such that xyz could happen".

It is also valuable to both form a hypothesis of how you think the code works, and then measure in the debugger how it actually works. Once you understand how these differ, it can be helpful in restructuring the code so it's structure better reflects it's behavior.

Time spent reading code is almost never fruitless.