A month ago I encountered a bug in LibreOffice Writer, which I use because it's really good, where track changes on a docx file wasn't working properly in a particular situation involving the deletion of tracked comments across saves.
QUICK! Find the line of code that caused it! Find the mechanism for creating a test case! I'll wait here.
If the open source project in question is one where users can be reasonably expected to be familiar with your code base, then "propose a fix with a unit test" is fine. But when it's a project whose nature means that users have little to no incentive to look at the code, then the reasonableness of this request decreases with the the size and complexity of the project. And, quoting BugsJustFindMe again:
Caring about FOSS doesn't fix bugs.
I'm a competent developer, but if I ran into a bug in the text-processing utility Pandoc, it's very unlikely I'm going to drop what I'm doing, analyze the source, pinpoint the problem, and submit a PR with a unit test, and oh yes, learn enough Haskell to do all this, and I don't think it would be reasonable to expect me to do so.