I'm trying to review a pull request on GitHub to a branch that isn't master. The target branch was behind master and the pull request showed commits from master, so I merged master and pushed it to GitHub, but the commits and diff for them still appear in the pull request after refreshing. I've doubled checked that the branch on GitHub has the commits from master. Why are they still appearing in the pull request?

I've also checked out the pull request locally and it only shows the un-merged commits.

Here's a good workaround. Use the Edit button when viewing the PR in GitHub to change the base branch to something other than master. Then switch it back to master and it will now correctly show only the changes from the most recent commits.