Blondie,
I can confirm that eating beef under-cooked is frowned upon in Spanish speaking congregations in the southern United States. Beef is the particular target since the meat has a red color as opposed to, say, poultry or fish, and the myoglobin in the beef makes the fluid look a lot like blood. That's the limit of my direct experience.
Am I aware of any official bans on eating steak rare? No. There exists a lot of confusion on the subject in my area and elders generally are cautious to issue official orders from the platform. However, much like my brother who was once an elder and prohibits his family from eating beef less than well-done, there are a lot of Witnesses who at least on an informal level believe it's improper to eat beef that retains a redish tint. Someone above posted a story about being the only one to order the steak less than well-done and getting a death stare from the other Witnesses.
It's mostly due to ignorance and lack of clarity on the subject from the WT, I think. At least half of the elders in my area were immigrants and had less than the equivalent of a high school education. If you cut into the meat and fluids come out that look a lot like blood, you tell people you can't eat it. I think it's really as simple as that.