Okay, it sounds like this is intentional and has been this way "forever", both on the individual commit view and the commit list.
The reasoning is mostly technical, but from a practical standpoint once the commit is on the main branch (and will likely be disseminated out to other branches) it makes less sense to consider the commit as belonging to a specific branch.
I see your point, but knowing the originating feature branch is pretty useful. Also, not that it is a reason for doing it in its own right, but github always shows the original branch for any commit, even after its been merged onto the main branch.
I'm needing this the other way around. I'm looking at a branch that has previously been merged into master and need to know what was committed to that branch between it being cut and being merged back. This would represent the total work that was completed on the branch. I'm at least needing to diff between the cut point and its most recent commit (currently obtainable by viewing its source, presuming the branch hasn't been deleted).