I kind of had the same experience with DL domestic flights. I was able to select comfort+ on two domestic flights.
Also something strange: I had a flight MSP-AMS on DL and for some reason my FF couldn't be updated to AZ Elite Plus, so I couldn't select any other seat. SOme hours before the flight though, the preferred seats on DL333 were free to select (even for non-status members) and so I did. At the gate, they informed that I had a seat change. How convenient!