That's probably by design. If you're turning off preview it's for "security" reasons and would be why it doesn't show who sent the message.
If it's by design, that's very unfortunate. I never want to see previews of the actual context of a text message, but I sure as heck want to know WHO sent it. Not sure why they suddenly changed this, but I'm hoping it's temporary.
Also, not sure why'd they'd hide the sender's name *only* in group messages. If it was a security thing, then shouldn't ALL text message sender names be hidden, rather than just those in a group thread? I think this is just a bug.