Upgraded, boarded, then told to give the seat back

I was flying out of DFW and got an automatic upgrade. Boarding pass showed the upgraded seat, I boarded, sat down, all normal.

After I’d been seated for a bit, a flight attendant came over and said the original passenger for that seat showed up, so I needed to move back to my original seat. I moved right away, no argument, but it was awkward.

What confused me is why issue the upgrade at all if it wasn’t actually confirmed. It just made the whole thing uncomfortable for no reason.

Honestly makes me think next time I should just stay in my original seat and ignore upgrades altogether. Curious if others have had this happen or if there was a better way to handle it.