You probably answered your own question, sounds like the app crashed so nothing was saved.
I've just done a test on Windows (WebView2) - I figured out that local storage is written to this file:
C:\Users\<username>\AppData\Local\<game name>\EBWebView\Default\IndexedDB\https_app.localhost_0.indexeddb.leveldb\000003.log
And that file is updated as soon as I call: LocalStorage - Set Item
So even if the app crashed, the data's already written to disk.
Maybe iOS handles it differently? Maybe if the app crashes, the OS rolls back the local storage to a previous version? (It looks like that 000003.log file contains all previous entries, so in theory the OS could roll back).