How do I stop my iPhone auto-deleting my files? I am using Notability on my iPhone (iOS 12). 

The files (about 5Gb) in the app are backed up on my iCloud. My iPhone often deletes the files stored on my iPhone without notifying me, which causes me serious troubles. There is no shortage of storage on my iPhone (44Gb of 64Gb used). 

How do I stop this behaviour?
Can you try disabling iCloud syncing for the said app, if it doesn't trouble you  much ?

iBooks for iPhone showed the same behaviour, automatically deleting books despite available storage. I had to turn off iCloud for iBooks .