The annoying thing about verification is when exchanges ask for unreasonable source documents that are even older than the exchange they were founded on.
I guess anyone willing to use a centralized exchange should just get ready to get annoyed all the time, as they will always come up with anything at anytime as a reason to freeze and withhold your funds.
The exchange might not be aware if the source of fund is no longer available or can't be accessed. When ever they detect any huge suspicious deposit from anyone they will definitely request for such documents, since they are also under some government regulation in other to protect their exchange and licensed.
I guess their T&C quoted below is one of the things they usually use to hold their customers.
We reserve the right at all times to monitor, review, retain, and/or disclose any information as necessary to satisfy any applicable law, regulation, sanctions program, legal process, or governmental request. We reserve the right to cancel and/or suspend your BitMart Account and/or block transactions or freeze funds immediately and without notice if we determine, in our sole discretion, that your account is associated with a Prohibited Use. Upon delivering a Final Formal Warning Letter through email, we reserve further disposal rights of any remaining funds (if applicable) in your BitMart account.