If you are using Project Online or Project Server 2016, you might have activated the resource engagement new feature. In case you are not familiar with this functionality, you can read this post I wrote when the feature was released. It comes in replacment of the resource plans. An interesting capability of the resource plan was to set wheither the capacity/resource utilization was calculated from the project, the resource plan or both (specifying a transition date).
Now where is this parameter? Navigating in the PWA capacity plans from the resource center, I couldn't find it. Got this answer thanks to the MVP distribution list, you have to set it using MS Project Pro, going to the project information. Note that the setting is available only for the published projects.
The "project plan until" still has the same behavior. This will trigger the utilization calculation from the project plan on a short term horizon until the specified date and then from the resource engagements.