I've been playing on my Android phone and on the web version but on several occasions I've randomly lost hours of progress for no apparent reason.

it's an issue with the syncing of your local storage to the cloud (or the other way arround)

see:
https://war.app/Forum/434291-syncing-between-devices
https://war.app/Forum/434686-bug-lost-progress-switching-web-app

should be better since the update https://war.app/Forum/436370-android-update-finally-live, but if you really want to avoid all risk, play WZI from 1 device, and don't switch to others...

Edited 2020-07-17 12:48:47