I think in general, bugs go unfixed in two scenarios:

1. The cause isn't immediately obvious. In this case, finding the problem is usually 90% of the work. Here it can't be known how long finding the problem is beforehand, though I don't think bailing because it's taking too long is a good idea. If anything, it's those really deep rabbit holes the real gremlins can hide.

2. The cause is immediately obvious, but is an architecture mistake, the fix is a shit-ton of work, breaks workflows, requires involving stakeholders, etc. Even in this case it can be hard to say how long it will take, especially if other people are involved and have to sign off on decisions.

I suppose it can also happen in low-trust sweatshops where developers held on such a tight leash they aren't able to fix trivial bugs they find without first going through a bunch of jira rigmarole, which is sort of low key the vibe I got from the post.