You can have a system where a gold certificate represents a particular gold ingot, and only issue paper certificates for the amount of physical gold you have in your vaults. But this is not the way most gold standard currencies operated.
You don’t need enough gold on hand to redeem each and every certificate issued for the same reason a bank doesn’t need enough cash on hand to pay out each and every depositor. Of course, back in the old days you would get runs on banks, when people feared the bank was going to fail they would rush to withdraw their funds, and this rush to withdraw funds would cause the bank to fail when they didn’t have enough funds to pay their depositors.
Likewise, the same thing could happen to a government, if people stopped trusting the gold certificates and everyone wanted to convert them to gold, the government wouldn’t have enough gold to cover the certificates. But then the government can just tell the certificate holders to fuck off, we are suspending redemption of certificates now, you’ll get your gold later. If people believed that pretty soon after the crisis was over gold redemption would resume, then the currency would take a hit. If people started to suspect that redemption would never happen, then the currency would nosedive as you said.
But if everyone rushed to convert their certificates to gold right away, there wouldn’t be any point to trying to create a gold standard currency, because no one would accept a certificate in the first place, they’d only take gold. You can only create a paper currency if people already trust the issuer enough to take the paper, believing that the paper could be redeemed in the future even if there isn’t enough gold to cover the paper. If no such trust exists, then your scheme of getting people to accept worthless peices of paper instead of real gold will fail.
Remember that even though a given government might not have enough gold on hand to redeem each and every certificate, they still have the power to tax, said power to tax being backed up by the police and courts and jails and army. So if a crisis occurs and there’s not enough gold, they can raise taxes, squeeze the peasants, shake down the merchants, confiscate the estates of the nobility, borrow from overseas, and get the gold they need. Later. If there’s a war or a revolution or a financial crisis, you aren’t getting your gold today. You’ll get it when the crisis is over, assuming the government survives the crisis or doesn’t default.
So yes, paper money was considered a huge risky innovation, even though the paper was backed by gold, because a gold certificate isn’t gold, is a promise to pay gold someday in the future. And that promise was only as good as the issuer of the promise. Low trust societies would never take paper certificates, or buy and sell on credit, or put money in a bank, because what happens when the promise gets broken?