In short, the logical branch names should match between the build configuration and the versioned settings root.
Your versioned settings root and the root attached to the PR configuration have different branch specifications, and the versioned settings root doesn't see the pull branches at all:
+:refs/heads/*
+:refs/pull/(*/head)
You can find more details about applying settings from branches in this article. Note that there are some limitation to what you can change in the settings of feature branches, they are also described in this article.
On a side note, you may want to use the Pull Requests build feature instead of manually extending branch specification to see the PR branches. The feature will bring some UI improvements related to PRs and it will also ingore branches of closed PRs. This is the recommended approach for building Pull Requests.