Closing a branch with one commit does not create a new commit

[Roberto Martignano]

Hello,
I'm Roberto Martignano, an Atlassian Consultant from GetConnected, an Atlassian Platinum Partner based in Italy.
Our customer is having some problems with BitBucket and SourceTree.
When they close a branch with only one commit, this is integrated in the develop branch instead of creating a new one.
In a first moment a working workaround was to set "do not execute fast-forward" in the settings (see pic), but after a SourceTree update it is not working anymore, and a rebase is always executed.

They're on version 3.4.5.

Do you have any idea of how we can fix this?

Thank you!
Roberto