From the Item Detail page of an AGOL Hosted Feature Service, one has the option to "Open in ArcGIS Desktop" or "Open in ArcGIS Pro". 

Pick the desktop one, and file downloads, and, if ArcMap is already open, the service will load in the currently open instance of ArcMap. 

Not in Pro 2.3. Pick "Open in ArcGIS Pro", a .pitemx file downloads, click on it, and it fires a new instance of Pro, which is bug. Why, if I already have an instance of Pro running, would I want another instance started just to view the service? The whole point of the AGOL functionality is you can, while having an instance of Pro running, click "Open in Pro", and that AGOL service would open in that instance. The reason, I suspect, is that Pro doesn't know which map to assign the .pitemx file to, so it defaults to a new instance, which doesn't make the behavior acceptable.