More info: I can upload files to shared folders when using the web browser on Windows 11, whereas it stopped working in OneDrive for Android, as posted above.
When checking the existing permissions of a shared folder, I noticed that despite my account being listed as having access, it did so by using a sharing link that anyone can use, which is not something I set up deliberately (I shared this folder years ago, probably around the year 2016).
I deleted the existing sharing links (for "can view" and "can edit") and created a new "can edit" one that only applies to named people, added my personal account and shared the folder. I received an invitation email in which it says that "This invite will only work for you and people with existing access". The link works, but it hasn't made a difference to the folder's appearance in OneDrive for Windows - it's still just a link, not a folder.
What do others see? Are your sharing links set to work for anyone, or for named people only? Does reconfiguring the sharing accomplish anything?