This sounds like something Pagan Min would do
[Update: Marek Zimny of G2A recently got in touch with Destructoid regarding this story. According to his comment, the company is not responsible for "any of these procedures" and is working towards obtaining refunds for affected customers.
"G2A is well aware of this process and will do everything possible to compensate for that. For users who have made transactions without G2A Shield, we will check if the corresponding merchant was responsible for the withdrawal of the code.
"If so, these users will get a compensation. We are carefully checking and verifying every transaction affected by this procedure, so we must ask users, without protection from G2A Shield, for their patience regarding upcoming refunds," Zimny said.
If you have protection with G2A Shield, you have the choice between a new code or a full refund. We are still waiting on comment from Ubisoft and G2Play.]
Multiple reddit users and members of Ubisoft's support forum have reported several cases of digital copies of Far Cry 4 suddenly being deleted from Uplay accounts. A cursory examination of these posts reveals the games were bought through different third party sellers, including independent eBay stores and discount digital storefronts G2Play and G2A.
In reply to complaints, Ubisoft has reportedly been advising customers to contact the seller, implying the license removal is not the fault of Ubisoft. However, there is a history of publishers canceling third-party Steam keys; Devolver Digital has publicly stated they will cancel any keys obtained through G2Play.
Devolver Digital games purchased on @G2A_com are not legitimate, not guaranteed, and not supported. We are actively canceling those keys.— Devolver Digital (@devolverdigital) May 14, 2014
Both G2Play and G2A are fairly popular stores, and both are based out of Hong Kong. G2Play's FAQ directly addresses the question of where they obtain their game codes. "All game codes/keys are picked up through major distributors, from their original retail boxes, and scanned up into our database." G2A, on the other hand, is an eBay-like code reseller, complete with user reviews and a referral program.
We've reached out to Ubisoft, G2A, and G2Play for comment, and will update this story accordingly.