Last week Google started rolling out a new version of the Android WebView with a bug which deletes site localstorage after app startup.
Roll-out was halted at around 50% and the Chromium team are currently working on resolution. More information to follow as it becomes available.
Symptoms: Any data stored in localstorage is missing after the webview has been updated and the app started up.
Affected WebView version:
Fix: Update to a newer version of the WebView.
More information: https://bugs.chromium.org/p/chromium/issues/detail?id=1033655