Git-flow finish feature merge does not obey fast-forward merge preference

I have the "Do not fast-forward when merging, always create commit" option in the git preferences turned on. I was surprised today when finishing a feature, which had only one commit and no other activity on the develop branch since the feature branch was created, that the merge was fast-forwarded on top of the develop branch instead of creating a merge commit as set by my preference.

Git-flow should obey this preference.