These types of projects undergo constant code-change/refactoring/re-architecture etc. If you don't add a test for your specific issue, there is a non-trivial change that it'd be broken again in some future release.
Its somewhat worse if an issue gets fixed, and broken again, vs. it being broken the whole time. E.g. with the former users have likely started to rely on the fixed behaviour, then will experience disruption when it breaks again.
if the author doesn't want to do it, the PR will likely remain abandoned unless an external contributor comes along to finish it, or a maintainer takes over.