When I select an active timeline to share, FCP says share is completed after a short time, when I look at the specific m4 movie file, I can see it decreasing in size. Next to it is an up arrow. After a minute or two, the file size is 0 bytes, down from 8 Mb. Any suggestions?
Check System Preferences>iCloud Drive>Optimize Mac Storage. Some cloud storage services will immediately upload the file, then show the file name with zero bytes. It then becomes a "streamable file". You don't typically want that for video files you're actively working on.
For further details see Apple and other documentation on how to configure iCloud Drive.