Numbers tells me I don't have permission to save

I'm editing several Numbers spreadsheets today, and Numbers keeps telling me that I can't save because I the file is inside a folder that doesn't belong to me (it isn't). I first noticed that it happened if I tried to save an open file after moving it to a new folder (to which I also had read & write permissions), but I cannot reliably replicate the problem. Plus, moving open files around has not been an issue on the Mac for almost 2 decades, so I'm a little confused. I have already checked ownership of all files and folders in question, and I have R&W access to everything.