Why do some files on iCloud suddenly force me to download again?
This is the correct situation if your Mac has Optimize Mac Storage enabled in the iCloud preference pane.



If your storage is under light pressure, I don't find recently used documents being released from local storage much if at all, but if you are running with a very close to full drive, the system may be removing the local copy before you like.



Above is an SSD under light pressure - lots of space for local backups, iCloud data to remain cached and space to add new files. Below is iCloud showing nearly 200 GB of data so if all of that were on my MacBook - my pressure would be constrained / close to full.



There's nothing sudden about this, though in my experience. Most people I assist with this are more bothered by the large amount of data that could be "purged" and isn't but I get your not wanting to re-download and wait for files.