Mobile App Bug Fix — User Data Issues (Flutter)

We have a live mobile app (Flutter) experiencing user data issues — data not saving, records disappearing, duplicate entries, or data showing incorrectly after sync. We need an experienced mobile developer to diagnose the root cause and ship a reliable fix. The Problem (symptoms we're seeing) User data occasionally fails to save or disappears after app restart Duplicate records appear after syncing (same item saved twice) Data changes made offline don't merge correctly when the device comes back online (conflicting/overwritten data) Some users report seeing stale or incorrect data after an update Occasional crashes or frozen UI tied to loading/saving user data Likely Root Causes (for context — please verify, don't assume) Race conditions in local storage or backend writes (e.g., offline queue flushing while new writes come in) Poor conflict-resolution strategy for offline-first sync (no last-write-wins or merge logic) State management issues (state updated in the wrong place, stale state re-render, duplicate writes triggered by rebuilds) Local DB (SQLite/Hive/Realm) and remote DB (Firebase/REST API) falling out of sync Missing unique constraints/idempotency keys causing duplicate record creation Improper handling of connectivity changes (online/offline transitions) What We Need You To Do Reproduce the reported data issues using our existing app and test accounts Investigate and identify the actual root cause(s) — don't guess, trace it Implement a fix that is safe for existing production data (no data loss during the fix/migration) Add safeguards to prevent recurrence (e.g., proper conflict resolution, idempotent writes, unique keys) Test across offline/online transitions and multiple devices/accounts Document what was wrong and what you changed Requirements Proven experience with Flutter in production apps Strong understanding of local storage (SQLite/Hive/Realm/AsyncStorage) and remote sync (Firebase/REST/GraphQL) Experience debugging offline-first sync and conflict resolution Comfortable working with an existing codebase (not building from scratch) Clear communication — you'll need to explain the root cause in plain terms Nice to Have Experience with Firebase Realtime Database / Firestore offline persistence quirks Experience writing data migration scripts that don't lose existing user data Familiarity with crash/error reporting tools (Sentry, Crashlytics) Deliverables Root cause analysis (short written summary) Fixed, tested code merged/delivered in a working build Notes on any data migration needed for existing users Brief testing checklist showing the issue no longer reproduces How to Apply Please share: A similar data-loss/sync bug you've fixed before (what was wrong, how you fixed it) Your experience with offline-first architecture and conflict resolution Estimated timeline and availability