I was about to post to say I haven't seen this issue. I've got two domestic itins (8 total segments) in the next two weeks and can see all seats in F/C+/C-.
After reading jamesteroh's post above it seems that it is because they are all connection carriers. Have a TATL in September and we are in F/D1 all DL metal, and can only see the cabin we're ticketed in.
So, more data points.