Unlike a mobile "custodial" wallet (like Coinbase or Blockchain.com), the Bitcoin Core client is a "full node" wallet. The wallet.dat file contains:
Because the internal index counter had moved forward, the user might assume the new addresses were saved. However, upon restarting and loading the wallet.dat , they would find that the keys were never written to the file. If funds were sent to those unsaved addresses, the Bitcoin would be unspendable—effectively burned—because the private keys never made it into the wallet.dat backup. indexofbitcoinwalletdat 2021