I guess that the Rennes Military Hospital was just using Front-Stalag 221 as its mail point. I have one letter from my Father from when he was captive and it shows Front-Stalag 221 in Rennes France. The NARA POW database is wrong about where he was held and I am trying to correct that. The NARA POW database does have Rennes Military Hospital but not Front-Stalag 221.
I will get a hold of the Red Cross and see what I can find out.
Arolsen should have a box full of data on Zweiglager (Branch Camp) 221 W at Rennes and the connection between it and the German military hospital at Rennes. In what appears to be a coincidence, there was a German Feldlazarett 221 located in Lons-le-Saulnier during 1943-44. That's a small spa or resort town in eastern France 68 km NW of Geneva, Switzerland.
If I had to make an educated guess about the relationship between Frontstalag 221 W Rennes and the German military hospital in Rennes, it would be that wounded Allied POWs were in the hospital for treatment but assigned to the Frontstalag. As soon as they were well enough to travel, American POWs were transported to Germany and interned in one of the Stalags that housed Americans and other English-speaking P/Ws.