it's about time! I noticed this yesterday but my first UA flight out of Asia isn't for a few weeks so I won't be able to test this until then.
For those who are panicking about what to do if you see your bag hasn't made it on the flight, don't panic. There's not a damn thing you can do about it on the plane. And you aren't going to take yourself off the flight because your bag didn't make it, are you?
I had this happen once within the past year or two and I was flying on DL. Yes, DL has had this functionality in their app for a while now. Their app tells you when it's scanned on and off a plane which is useful. Long story short, I figured out my bag didn't get checked onto the flight I already boarded. About halfway to my stopover point (ATL) the app showed my bags already re-routed and on a flight through another connecting city to my destination. Sure, they still arrived late. But they were getting there and I could see that data on the phone. So, no big deal.
-RM