My account was locked after I sent out payments by using AMEX credit card (not ACG). The point is Venmo is extremely sensitive to AMEX-related payment now (including both CC and GC). It seems like their system will automatically flag the transactions related to AMEX CC/GC. Their account specialist partially confirmed this mechanism with me in the chat.

During the unlock process, Venmo asked for the photo of my card, including the first 4 digits and the last 4 digits of the card number.

I guess they are trying to find a way to stop the ACG loophole.