a) you should not be the owner (to avoid pet projects that are not actually useful) of the project or at least not the sole owner
b) ideally it should be some high impact projects that have little to no corpo sponsors as opposed to something like React
c) if your contribution is not merged in, it should not count as "work done"
React is also now owned by the React Foundation, so I also don't see why it would be problematic to contribute to it now that it doesn't (seem to) belong to Facebook anymore.
If the project is truly open source and widely used by the community it shouldn’t matter if it is or was associated with a corporation. Contributing to it helps the public who use that project too.