Andre, you are correct in stating that a Pixel has only internal storage available and I don't use external storage to access my music, but both you and Geiler Heinz were also correct in pointing me to the Library Scanner section of Poweramp. I noticed in this section that Auto-scan, Initial Scan, USB Disconnection/SD Card Mount, and System Media Scann/MTP were turned on. I decided that of these the only ones that would be the most likely responsible for the behaviour I was seeing were Auto-scan and Initial Scan. Of these two, Initial Scan seemed the most likely given that it would likely see a reboot as Poweramp starting for the first time and run a scan. So, I disabled only this option to test my hypothosis and rebooted the phone. Upon reboot the Poweramp Widget came back up with my playlist showing as I had left it before the reboot and the only thing missing was the album cover but pressing the play button and starting the song loaded the album cover and after that all the other songs, album covers, etc, on the playlist worked fine as well. I can live with this. Thank you both for working this through with me.