When doing a merge from one branch to another using the "merge fetched" option, the merge will be automatically be committed even if the "commit merge immediately" checkbox is unchecked.

In a previous version of Sourcetree, leaving this checkbox unchecked would allow me to preview the results of the merge and then manually commit them and leave a commit message.  Currently, I cannot leave a commit message when doing a merge via merge fetched as it is automatically committed even with this box unchecked.