I have been testing some things and it does look like an FMC issue (as suggested by @PZL104), possibly the integration of the flight plan created from the World Map and the FMC.
What I have been trying is to just select the departure airport and gate from the World Map then Fly. I used SimBrief to generate a flight plan, then manually enter everything onto the FMC (departure airport, arrival airport, all of the waypoints, etc.).
With flight plan from World Map: all 4 tries the co-pilot changed destination
With the manual flight plan entry: out of 3 tries, 2 did not change destination. I may have setup something that is the actual trigger on that 1 try however I have not yet pinpointed which one is it.