"One of our PC retail distributors informed us that some of their allotted Steam keys were stolen," the company wrote on its Steam Community page. "We believe these keys were then resold to multiple companies, with no payments going to either Valve or the retail distributor. Steam were immediately informed and have now revoked that set of keys."
Rebellion acknowledged that those affected may not have been aware that the key they purchased was illegitimate, and is thus offering the game's "Target Hitler" DLC for free. Rebellion themselves could not offer refunds, but strongly encouraged those who have found their access revoked to contact their seller to receive one.
For more details on how to get the free DLC and the full post of Rebellion's explanation, check the game's Community page.