Did not know of this until I saw something on the B-24 Liberator fans FB page. I was skeptical, but I looked it up, and yes, there were some US POWs at a concentration camp. This was not, however, a Wehrmacht policy, it appears to have originated from a rogue commander. What's amazing is the circumstances that allowed them to get out:
At last, the U.S. POWs of Buchenwald Talk
At last, the U.S. POWs of Buchenwald Talk