Ideally, your library code that you're pulling in has some unit testing to demonstrate that things are working as they should be. (If not, consider adding unit testing! It's really useful!)
If that is the case, then you can isolate the likelihood of an issue as either in the library (because unit tests fail there), or in the project consuming the library (because unit tests succeed in the library).
Without testing, it's hard to have a ton of confidence in where the problem lies—which is exactly the problem you cite. And while a monorepo (or just a standalone repository with no separate libraries, which is frankly an easier setup to manage than monorepos or multirepos!) may make debugging a bit easier, it's not going to give you much more confidence in your code.
Once your organization outgrows the paradigm where you just have N standalone projects with N completely separate code bases, and you do need to commit to either a multi-repo or a mono-repo configuration, it's really, really helpful to have unit testing to allow you to isolate where issues are.