I just ran into this on a new Moto e4, and I think that I have accidentally stumbled on a solution.
Problem: Pocket app disappeared from home screen and/or app folder after restart.
Solution (I don't know why it works):
1. Enable Developer options
https://www.digitaltrends.com/mobile/how-to-get-developer-options-on-android/
2. Reformat sd card (in my case a 32 Gb Samsung Class 10 microSD)
3. Reinstall apps, then move everything possible to the sd card. This freed up about 2 Gb from internal storage.
4. Restart. Icons for all apps that were moved to the card disappear.
5. Disable Developer options
6. Reformat card as internal, move everything possible to external card
7. Restart. All the app icons that previously disappeared are now visible even though they are shown as being on the Samsung SD card.
I have restarted my phone several times since, and nothing has disappeared yet. Fingers crossed.