I ran into the same problem just 2-3 days ago. First I thought that the file system is not working with “project” chats but then I figured out that its tied to GPT 4 mini and GPT 4. I have Plus membership and when I use my project chats I use GPT 4 and for normal chats GPT 4 mini. When the upload worked with GPT 4 mini I switched in the same chat to GPT 4 and it was not able to see the files again, I switched back to mini and it could again.
its 100% a bug because I did the following. I uploaded my project files to mini asked it if it can read it, it said yes and gave me a summary. I switched to 4o and uploaded character information of a story i asked 4o for information about a certain character and it had no information and also confirmed it cannot see files. I switched back to mini in the same chat, didn’t change anything, asked for the character again and it gave me the information about the character by reading the uploaded files. So something is weird here.
I use the windows app btw. no browser interface used.