On SourceTree for macOS, newest version (2.5.1), when trying to merge with right click-> "merge X into Y" -> after the confirmation pop-up shows, there is no option to force a merge commit. On SourceTree for Windows (v 2.x) there pop-up is the same, except for the 'force merge-commit' checkbox.

Therefore, a fast-forward is done always when possible. This is incorrect behavior if, for example, we are following the git-flow rules.

If I merge a branch through the "merge" button on top of view, I can force a merge commit, so that's a temporal workaround, however not really comfortable.