It's pretty uncommon. Most of my teammates' code is close to my standard -- I am referred to as the "technical lead" even though it's not an official title, but teammates do generally try to code to my standard, so it's pretty good code. If I'm doing a one-off bug fix or trying to understand what the code does, usually reading it is sufficient. If I'm taking some level of shared ownership of the code, I'm hopefully talking a lot with the person who wrote it. I'm very choosy about open-source libraries so it's rare that I need to dig into that code at all. I don't use any ecosystems like React that tempt you to just pull in random half-baked components for everything.
The conflux of events requiring me to debug someone else's code would be someone who wrote bad code without much oversight, then left the company, and there's no budget to fix their work properly. Not very common. Since I usually know what the code is supposed to be doing, such code can likely be rewritten properly in a small fraction of the original time.
