> unless fixing a bug requires a significant refactor/rewrite, I can’t imagine spending more than a day on one

Race conditions in 3rd party services during / affected by very long builds and with poor metrics and almost no documentation. They only show up sometimes, and you have to wait for it to reoccur. Add to this a domain you’re not familiar with, and your ability to debug needs to be established first.

Stack two or three of these on top of each other and you have days of figuring out what’s going on, mostly waiting for builds, speculating how to improve debug output.

After resolving, don’t write any integration tests that might catch regressions, because you already spent enough time fixing it, and this needs to get replaced soon anyway (timeline: unknown).