Wallet balance corruption under concurrency

An accounting reconciliation flags that aggregate 'credits remaining' across wallets is drifting LOWER than the sum of individual debits/credits should allow, money is quietly disappearing from balances under high concurrency. Dashboards: no errors; the wallet service reads a balance, computes the new value in app code, and writes it back (UPDATE wallets SET balance = ? WHERE id = ?); during traffic spikes, concurrent debits on the same hot wallet (shared team accounts) interleave; the DB is Postgres at READ COMMITTED. There's no row lock or atomic decrement. How do you triage this corruption, stop the silent balance loss, and recover the lost amounts?