Cleaning out iCloud

“Cleaning” appears when your device is clearing the app’s cache, which is a temporary storage area for documents and other data. You don’t need to do anything: just wait for the word to go away, which means the process is complete. No data is lost during cache cleaning, because your documents are stored on iCloud.
Apple Support

Ridiculous. When your iOS device fills up, your apps create space by deleting every locally cached iCloud file.
Not just the files you didn’t open up for a month, not the biggest files first. No, every document that’s synced with iCloud is erased from your device.

Imagine the following scenario: you’re at the Airport ready to board a plane. On your Apple are all the presentations you’re planning to work on. You quickly download a few podcasts and David Spark’s Email Book while waiting in line. And then your iPad starts cleaning out your cache since storage is filling up. Say goodbye to working on the plane cause your presentations where just deleted. Well, at least you still have a very good book to read.

Apple, please enable intelligent caching of iCloud files. Just allow me to star the items I always want on my device for example.