Editions: Daedalus Rewards
On Shelley Incentivised Testnet Daedalus Rewards showed that a transfer was confirmed but then sometime later the transaction disappeared from transaction history and the funds reverted to the wallet that sent them.
The first thing to understand is that we are dealing with a blockchain. So it is possible that the recent transaction history changes, goes backwards, then forward, then backwards again etc ... Big jumps should be fairly rare but they can still happen. In this case, what looked like a confirmed transaction was not confirmed and reverted. Daedalus Rewards is configured to wait a certain amount of time to confirm a transaction, and this should work in most cases. Occasionally it will not work, because the transaction gets cancelled in the blockchain after a considerable amount of time.
To a certain extent, this is expected, especially as this is a testnet environment. The only thing to do is to try your transaction again.