A known issue with users with some type of a cleaner/optimizer app on the device is that it's set to remove "large/big" files automatically.
The dictionary database is one large file that provides offline access, so these "optimization" apps directly target it every day. The simple remedy is to change the setting of the cleaner app.
Cleaner apps usually have access to remove Data and Cache as well. You may need to disable the clear data function if that's done automatically, which will also wipe out the offline database of the dictionary.
Samsung's "Device care/Storage booster" & "Files by Google"( an example app) , as well as "Phone Master" are commonly known to cause this.
Another possible reason might be a default phone cleaner/optimizer and the third-party cleaner apps they may have installed.
If you have a Samsung device, you should have an Auto optimization feature built in.
Please follow these steps to turn off this feature:
1. Go to Device Settings > Biometrics and security
2. Scroll down to the bottom of the screen and tap "Device care"
3. Tap on "Storage" and then tap on "Storage booster"
4. Switch off the "Audo delete unnecessary data" feature.
Disclaimer: Some Samsung devices, have "Device care" as a separate feature. From Device Settings, go to "Battery and Device care" and tap on the three dots at the top right corner of the screen. Tap on "Automation" and swtich it off the "Auto optimise daily" toggle switch.
If you are using a Google Pixel device, please follow these instructions:
- Go to Device Settings
- Tap on "Storage"
- Turn off the "Smart storage" feature
Also, if you have downloaded the "Files by Google" app or any similar cleaner app, it most likely targets and deletes the Offline Database every day as well. You can check the app's settings and switch them off accordingly.