This is the expected behaviour, this use case should not lead to a conflict. There is no “real” merging done, since the changes from the branch can be simply applied to master as if the other branch never existed (this is called a “fast-forward merge”). Before merge:
--A--B <-- master \ C--D <-- branch
After merge:
--A--B--C--D <-- master
If you wanted to see a conflict, you would have to create a history like this:
--A--B--D <-- master \ C--E <-- branch
Now if the commit D “touches the same lines” as C and E, there could be a conflict when you try to merge back to master.