GitKraken have what seems to be a nice built in UI to work with git flow. You can start and finish features, hotfixes and releases on a click of a button.
I was testing it in a local repo and I started a feature and made 3 commits to it. When I finished the feature (by clicking the GitKraken's finish button), the branch was merge with fast forward into the develop branch.
As far as I know this is not the expected behavior of git flow.
Am I missing something here?