Yeah, I didn't even include technical debt into my list, but it comes up so often as well. Chances are someone else already saw the problem, considered fixing it, then saw the mountain of technical debt blocking a fix, and decided to ignore the problem in the end.

For me, thankfully, folks have been understanding so far, when a bug fix takes significantly longer than expected.
But we also explicitly don't estimate bug fixes upfront, because of what you say. You have to diagnose the bug to figure out what solution is required. And the solution can range from a one-line-change to a refactoring of half the codebase. Good luck estimating that...