Numbers app on iPhone can't open files

Numbers app on iPhone can't open files from iCloud Drive, even files that have just been created on the phone by the Numbers app, using the Create Spreadsheet option.

Very useful error messages:

1) When opening an iCloud Drive spreadsheet that was created on a Mac, I get this: "The file filename couldn't be opened." The file has a little cloud on it, so I tried using the download option. The file didn't download. It can't be copied, shared, or moved either, presumably because it's stuck in the cloud. I can open the file with Mac Numbers or with a Mac browser logged into iCloud. A few days ago, this was not an issue.

2) When opening an iCloud Drive spreadsheet that was created on the phone itself using "Create Spreadsheet," I get this: "Filename can't be opened right now. Numbers couldn't read the file. You can try restoring to a previous version."

The phone is connected to the web using my Wifi, and, just in case, Numbers is also allowed to use cellular, so this is not a connection problem.

I am using an iPhone 7 with iOS 13.2 and Numbers 5.2.

-------------

More generally, why is so much Apple software so pretty and so lousy? It seems every time I relent (I avoided iCloud for years) and try their creations, I run into stupid problems that take more time to solve than the actual work I'm doing. Yeh yeh, no need to answer that.