You are a law-abiding citizen and you have no idea how the bitcoins that you own now were used before. However, there is always a chance of getting "dirty" bitcoins that were used by criminals to buy drugs or weapons, or were used in money laundering.
What to do if the crypto exchange has frozen your account?
How many cases have there been that the exchange has frozen "illegal" bitcoins?
Is it true that a cryptocurrency exchange could freeze any account with suspicious bitcoins?
Yes, subject to terms & conditions and applicable law
What to do if the crypto exchange has frozen your account?
Contact the exchange.
Best to store Bitcoin in a standalone wallet
If an exchange decides to seize your Bitcoin, that's a civil matter between you and them. It happens not infrequently, as most exchanges use tools with questionable reliability to reduce their perceived risk. Generally speaking as a regulated business, it's their job to attempt to reduce any instance that may be money laundering or identity theft.
