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.
It sounds like you created this backup before you encrypted the wallet. When you encrypt it, it creates new private keys. If it was funded before you encrypted it, those keys would still be in there. But if you only funded the wallet after you encrypted it, the backup you made before that became useless.