if a library is used by multiple services, and gets an important bug fix, each service using the library needs to update to get the fix
these are sequences of changes, not literally at the same time or requiring deployment coordination, but when this happens a lot people start asking about monorepos
You have to make the change to service A and B, and then test both and deploy both. You haven't saved any time or effort in a monorepo.
A library needs updating, you still have to update it and then test and deploy every service that relies on it independently. Again you haven't saved any time or effort. In fact you've made it worse, because if those services are maintained by different people, you just forced them to test and deploy on your timeline and priority, not theirs. You actually made the coordination problem worse.
The thing microservices is supposed to solve is eliminating the need to make atomic changes across services. There is an inherent overhead in doing that, like making your service support both new and old APIs as long as any service exists using the old API. This is the cost of microservices.
The tradeoff is you need to invest into repo scaling, much like a backend service that needs to invest in scaling itself too.
In the library scenario, I know tooling like bazel ensures that’s one changeset, not 3+. Tests run for both the changed library and all consumers of it in the same pass. You’re right that it might loop in others for review who weren’t expecting it, but i think that’s the same in a polyrepo approach.