An Open Letter on the Ubisoft forums has complaints from gamers who have had their copies of Far Cry 4 removed from their UPlay game libraries, apparently because they were unlocked with game keys from unauthorized sellers, with a number of posts identifying G2A as where they purchased the game. Since the keys themselves are legitimate, this leaves impacted gamers wondering why they are being punished, but apparently this clawback was executed before all the facts have been gathered, as Eurogamer has the following vague statement from Ubisoft:

We regularly deactivate keys that were fraudulently obtained and resold. In this case, we are currently investigating the origin of the fraud, and will update customers as soon as we have more information to share. In the meantime, customers should contact the vendor from whom they purchased the key.