I have had this happened a few times now and have written the GS agent about this:

upgraded and received text and new boarding passes on my app, then gate agent sat someone else in my seats and FA wanted nothing to do with my requests to look into it . All these happened while I took my assigned Y seats and the text came in just before door closed.

Got 7.5K miles for the latest incident. (This is the domestic portion of an international itinerary supported with PPS which the international segment cleared)

Previous incidents related to domestic itineraries only but I never complained. This latest incident just tipped over the scale for me.