I think it is more to do with when the flight opens for check-in than any system issues. I had UA issue NZ BP's at LAS on a LAS-LAX-AKL-KKE flight two months ago, and tag my baggage all the way (had to re-check the bags in AKL though). I did find that NZ ripped up the UA BP's in LAX and replace them with their own though...

I also find that UA sometimes insist on issuing their own BP's to replace those issued by other carriers for their flights.