It's release candidate 15.1, so yes, it's a beta. But I think you found the answer... I have an Apple Watch.Yes, I checked available updates and still only see 15.0.1 on my 13 Mini. And still cannot get messages read to me unless I either have it set to show previews, or have get the phone unlocked (which doesn't happen when it's in my pocket).
There should be an additional option to break through the security just for that. "Allow Siri to read messages when phone is locked: Yes/No"