I entered my compressed WIF private key as an input and the uncompressed address matches my address with the BTC balance. The uncompressed WIF private key starts with a 5 and is 51 characters long.
I knew private keys can produce 2 different legacy addresses, but I didn't know the blockchain website messed this up.
Does this mean that I can use this uncompressed WIF private key to sweep my bitcoin to a new wallet?
Yes. Or, you can use it to sign offline signing from Electrum on offline Tails. It depends on how much money is involved: if it's small, I'd risk it. If it's a lot, I wouldn't risk it.
After you've moved your Bitcoin out of that address, you should read
my Fork claiming guide. The numbers are outdated, but there's still value in there. But don't even think about this before moving your Bitcoin, there's no need to risk losing them to a compromised Fork wallet.