The real problem here is that the contributor (presumably) didn't give a heads up before implementing the feature. The maintainer also stayed silent for nearly

The real problem here is that the contributor (presumably) didn't give a heads up before implementing the feature. The maintainer also stayed silent for nearly 4 months. This incident shows how important it's for them to communicate. Other than that, there is no problem contributing to an open core project if it gets accepted. The community gets to benefit as well. On the other hand, a CLA in such a project is something you need to avoid by all means.