That will happen if sometime during those years the bitcoin was send to another adress. You synchronize the blocks starting from block 1. At one point the synchronization is at the block where your adress received the coins. Btocoin core will then show your coins in the balance. After some time the block wehre the coins were send away will be synchronized, so the bitcoins will disappear again. The best way to see if your BTC is still there is to check also in mempool.space .