This happened to me too.
I stupidly Googled for "normal fixes" for Photo Stream problems so didn't see this thread before taking more severe options.
I did the usual "turn off and turn on Photo Stream", and "turn on and off iCloud" (goodbye huge existing Photo Stream) - and lots more.
Fortunately I have my photos synced to my Mac - because that's the only way I know to sync smart folders (which I use a lot with keywords - even though I know that seems to be a dead end in the Apple world) - so I haven't lost all my photos.
Because the next option I tried was the traditional (and fortunately not something I've had to do for years) reset phone.
I restored from a backup (note to self - don't rely on the "Back up your most important data..." option - go back to "all") and then, when my Photo Stream still didn't work, I set up my iPhone from new (admittedly - there's still all the iCloud stuff that downloads with most of my "life" in).
This STILL didn't work - so don't bother with that.
THEN I found this thread - darn it all.
Valuable tip if you've been using your phone "just the way you want it" for a long time (and always upgrade phones based on backups) - make a Note in Notes with the layout of your phone in. I have one in text form (row by row, contents of folders, etc) but if your layout isn't complex, screenshots would work too. This means if you ever have to rebuild your phone, your muscle memory still works - you can put everything back where it's supposed to go.
Ironically, there also seems to be a bug in iOS that saves screenshots to your Photo Stream when you don't save (you take a screenshot, copy it to paste somewhere, then click delete) - the screenshot doesn't go to your Recent Photos but it does go to your Photo Stream - and THAT still happens - so the only part of my photos that goes to my Photo Stream is the bug 🤦🏻♂️
I have tried the change format - but that didn't work - so hopefully this bug gets fixed soon.
Good luck everyone!