How to stop Numbers files from appending filename extension

MacOS Sequoia 15.4 using Numbers 14.4. Some, but not all of my Numbers files keep reverting to show filename extensions.

In Finder, I have the Show All Filename Extensions box unchecked. It has remained unchecked and most of my files show that the Hide Extension box is checked when looking at Get Info.

However, on two files the Hide Extension box is unchecked. When I check it, after a couple of minutes, it unchecks itself. These files are old and have saved for years without issue, but now are defaulting to show a filename extension despite the Finder settings. This started about a week or two ago.

I have compared Get Info between "normal" Numbers files and these other files that show extensions. All info is near identical except for the Hide Extension box. They are located the same (on Desktop, shared in iCloud).

I can create a new Numbers document and copy paste each tab from the Extension showing file to the new file. I then delete the old file and just save the new one as the old file name. Everything is now fine; that's the workaround. But I'd really like to know why these few files are refusing to keep the Hide Extension option.