Why did my Bitcoin balance change after a transaction even though I sent less?

The answer

A Bitcoin spend typically consumes an entire selected UTXO and returns the remainder to a change address controlled by your wallet, minus the fee.

What to do next

Check the transaction outputs and confirm the apparent extra destination is a wallet-owned change address before assuming money was stolen.

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Does the change address require a new backup?

Modern deterministic wallets usually derive change addresses from the existing recovery phrase.