Updated content does not save in container

[-dp-]

I'm attempting a document management system with externally stored containers. When I open a stored file, change the content, then save and close the file, the container does not update, with the result that my changes are lost. In one experiment, it created another copy in the file system, (filename, and filename-1). I'm not attempting any versioning. I just need to open the file from the FM container, modify it, and save the changed file back into the container, leaving only the changed file in the file system. The only way I've found to reliably save changes is to open the file in the container, save a copy to the Desktop, delete the file in the container, insert the file from the desktop, then delete the file from the Desktop. Clearly, I don't understand this feature. Could someone please educate me? Thanks.

macOS 11.2.3

FMP 19.2.2.234