Similar situation. Client tried purchasing an item. Signified (app that works with Shopify for fraud prevention) flagged it was automatically cancelled. I checked my order read such.  Client then tried two more times and same thing. I wound up manual...

They’re doing absolutely nothing about these issues, and they are making. Fortune off of cancelled orders! Very upsetting. 

Client tried purchasing three times and all three were immediately cancelled, and marked cancelled on their orders. Meanwhile even after chatting with Shopify suppprt to ensure the client would not be charged and the funds would not be released to me...