I had a backup of my wallet.dat a few years ago, which was encrypted for sure.
But when I try to reload it now, it shows 0 balance and no historical transactions after the whole blockchain sync, and it is weird that the option of encrypt wallet is available.
With any Bitcoin wallet software, you must connect your Bitcoin wallet to Internet, sync it with Bitcoin blockchain to have a correct balance figure. Exemption is cold wallets that must be kept offline and don't connect to Internet.
Your information is vague and very confusing with me. Would you mind explaining how did you open an encrypted wallet if you don't have a wallet password, and how did you see the balance without unencrypt it for opening it, please.
Even with SPV wallet like Electrum wallet, you must open your wallet (with or without wallet password, depends on how you set up your wallet previously), you must have a green icon that signals your wallet is already fully synced, then you will see the wallet balance shown by Electrum wallet software.