Errors adding tracks and loading web player

[foxli]

I've been having a problem for a couple of weeks with the web player. I've added a few tracks to my starred playlist recently that show up right after I add them, but when I come back a day later, the tracks are gone. I know they're not gone from Spotify because I either search for and find them again or see  them on another playlist that I added to at the same time. And I think the web player is reading them as being there, even if they don't actually show up in the liste where I can click them, because if I scroll down a bit and try to play or queue a song, the song below it will be the one that's selected. This happens when I double-click, use the right-click menu, or the dot menu to the right next to the track time. This is happening on multiple computers, on Windows 7 and 10, and on Firefox and Google Chrome.

Similarly, tracks will disappear from a spot further down on the Starred playlist and reappear elsewhere. it's not a sorting problem, since you can't sort in the web player. They're all supposed to be chronological.

I've done nothing else to change my behavior or browsing on Spotify before or since this problem came up, so what's going on?