I mean that the company pays someone (like an existing employee) to maintain their internal fork and contribute patches back upstream.

Oh, most companies will pay someone to maintain an internal fork, but hardly any will contribute back. Sometimes that's due to lazyness, sometimes it is the idea that nobody will care for the company internal stuff, but most of the time it is outright forbidden to share internal IP even when that comes in the form of patches to open source code.

In my experience it is safe to just ignore that case and not care about corporate convenience when starting any open source project.